Monster Cookie Bars
Loaded with oats, peanut butter, and M&Ms, these chewy, naturally gluten-free Monster Cookie Bars require only ONE BOWL yet yield a BIG BATCH, making this the perfect quick and easy recipe when you need a bunch of sweet treats for a party or celebration!
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Today I'm sharing one of my favorite sweet treats for feeding a crowd. These Monster Cookie Bars are quick and easy to throw together in just one bowl and they yield a big batch. As a bonus, they don't contain any flour and can be made naturally gluten-free. Once you try it, this is a recipe that you'll make time and time again!
What Are Monster Cookie Bars?
You've all heard of Monster Cookies, right?
They're known for being nice and chewy thanks to rolled oats. They have an unmistakable (and yummy) undertone of peanut butter. And this version is loaded with not one, not two, but THREE different confections: M&Ms, chocolate chips, and peanut butter chips!
Seriously, this is a recipe that you definitely want in your arsenal. But why do I love it so? Let me count the ways.
Totally Customizable
First of all, the M&Ms in these bars make them fun to customize for various holidays:
- Add them in the colors of your favorite team for sporting events.
- Use Reese's Pieces in place of M&Ms for pretty fall colors.
- Or buy seasonal M&Ms in color schemes -- from Halloween to Christmas to Valentine's Day to Easter to the 4th of July -- for festive, holiday-themed cookie munching all year long!
One Bowl + Sheet Pan = Easy Clean-Up + Big Batch!
Next, this recipe is super quick and easy to whip up. The dough comes together really quickly -- there's not even any flour to measure out! -- and in the process of making them, you only end up dirtying a few measuring cups/spoons and just one bowl.
Furthermore, these bars are baked in a large sheet pan that results in a ton of bars, meaning that this recipe is perfect when you need a large quantity of treats in one easy batch.
***Just be sure that you're aware of any potential peanut allergies before sharing these treats at a party or celebration. It may not be obvious to everyone that they contain peanut butter.***
Now to back up just a bit, I'm notorious for turning classic cookies into bar cookies. Take, for instance, my Chewy Snickerdoodle Bars. You see, when I need to bring a baked treat to an event, I've found that it's way faster to spread batter in a pan and bake as opposed to hand-forming dozens of individual cookies. So I took my classic Monster Cookie recipe and did just that. I first baked the batter in a 9- by 13-inch baking pan. Unfortunately, my bars turned out pretty thick, which resulted in them being a bit too underdone in the center and a bit too overdone around the edges of the pan (not that we didn't happily eat the rejects, mind you).
Anyway, I decided to scrap that approach, tweak my ingredient amounts, and try, try again, this time taking a hint from my favorite brownie recipe and making these bars in a large, rimmed half sheet pan for more even baking. And ding, ding, ding...it worked! The cookies were perfectly soft and chewy without being too gooey in the middle or too crunchy around the edges. In other words, Monster Cookie Bar perfection!
But my final (and probably most important) reason for loving these Monster Cookie Bars is that they are freaking delicious. The chewy texture, the variety of goodies within, and the peanut butter goodness make for one tasty treat.
Ingredients
So what does it take to throw together this Monster Cookie Bar recipe?
One ingredient that it doesn't contain is flour! That's right...so long as you use GF oats, these treats can be made naturally gluten-free!
Here's what you do need...
And now for a few ingredient notes:
- Peanut butter. I typically opt for creamy, but you may use crunchy if you prefer. And regular peanut butter will work just fine, but I have used natural/organic peanut butter (just peanuts + salt) with no issue. If you have a large jar of PB, you can simply measure out two cups. But if you are buying ingredients specifically for this recipe, you can just buy a 16-ounce jar of peanut butter and use the whole thing.
- Butter. It needs to be softened. I usually use unsalted, but salted will work as well.
- Rolled oats. You must rolled oats! Not only do they lend these bars their chewy texture, but they also measure out differently than quick oats. And you must use exactly 18 ounces of them! It's easiest to just use an entire 18-ounce tub of rolled oats, but you can also use a kitchen scale to measure them out.
- And again, if you need your cookie bars to be gluten-free, be sure to use gluten-free oats.
- The add-ins. I love adding a combination of goodies to my Monster Cookie Bars...namely, M&Ms, chocolate chips, and peanut butter chips. To make sure my treats are nice and colorful, I reserve half of the M&Ms for sprinkling and pressing into the top of the bars before baking. You can use regular-sized M&Ms or mini M&Ms or, as mentioned above, even seasonal/holiday M&Ms to fit the time of year.
How to Make Them
Making these bars is quick and straightforward. And because the recipe yields a big batch, you will want to make it in a BIG bowl with a wooden spoon or a BIG stand mixer.
- Mix together the eggs and the sugars.
- Beat in the salt, vanilla, peanut butter, and butter.
- Stir in the baking soda and the oats.
- Mix in the goodies!
- Plop the chunky dough into your greased sheet pan. THE SIZE OF THE PAN YOU USE IS VERY IMPORTANT to prevent overflow...you must use what's known as a rimmed "half sheet pan," measuring 18 x 13 x 1 inches.
- Now here's another important step...you'll want to firmly press and pack your cookie dough into an even layer. If the center is a little higher, that's okay, but make sure the edges of the dough are below the edges of the pan. Again, this is to prevent overflow. Sprinkle with additional M&Ms and press them into the surface.
- Pop your pan in the oven and bake until puffy and very light golden brown. These Monster Cookie Bars are intended to be moist and chewy, so be careful not to overbake!
And that's it. You've just made a big batch of soft, chewy, decadent cookie bars!
A Summary of How to Prevent Overflow
I've had a few comments over the years about these bars overflowing the pan. This has really stumped me, as I have made this recipe dozens and dozens of times (and in different ovens, no less) and I have never had this issue nor been able to replicate it.
That being said, I have done my best to trouble-shoot what could have caused this problem. If you have read all of the above info, you've already seen my warnings, but I wanted to summarize them again here just in case!
- Use exactly 18 ounces of rolled oats (not quick oats). Weigh them with a kitchen scale if you are not just using an 18-ounce tub. This recipe used to give the conversion that 18 ounces was equivalent to about 5 ½ cups of rolled oats...but I hesitate to recommend that anymore because I worry that measuring out different brands of oats may have ultimately resulted in using different amounts by weight, and this could be why a few people experienced overflow.
- Make sure you are baking your bars in a true "half sheet pan" that measures 18 x 13 x 1 inches. Here is the one that I use.
- Press the dough firmly into the pan. You are aiming for a packed-down, even layer, but with the edges of the dough pushed down below the edges of the pan. This allows a little room for rising without overflowing.
So the next time you need a large, effortless batch of cookies for sharing, look no further than these Monster Cookie Bars!
They're colorful, they're fun, they're totally addictive.
And I promise that your big ol' cookie sheet will be empty in no time. 😉
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Monster Cookie Bars
Video
Ingredients
- 4 eggs
- 1 ½ cups sugar
- 1 cup brown sugar
- ¾ teaspoon salt
- 1 tablespoon pure vanilla extract
- 2 cups (16-ounce jar) peanut butter, creamy or crunchy
- 5 tablespoons butter, at room temperature
- 2 ½ teaspoons baking soda
- 18 ounces (1 small tub) old-fashioned rolled oats
- 1 cup M&Ms
- ½ cup semisweet chocolate chips
- ½ cup peanut butter chips
Instructions
- Position rack in center of oven and preheat to 350°F. Generously butter a 18x13x1-inch rimmed half sheet pan. Set aside.
- In a very large bowl (using a spoon or an electric mixer), mix together the eggs and sugars until well-combined. Beat in the salt, vanilla, peanut butter, and butter until smooth. Stir in the baking soda (make sure there aren't any lumps) and the oats, then add the M&Ms, chocolate chips, and peanut butter chips and mix until just combined.
- Spread the batter into the prepared baking pan, smoothing the top into an even layer and pressing down firmly. Bake for 16 to 22 minutes or until edges are very light golden brown and center is set. Allow to cool in the baking pan before slicing and serving.
Equipment Needed
Notes
- Use exactly 18 ounces of rolled oats (not quick oats). Weigh them with a kitchen scale if you are not just using an entire 18-ounce tub. This recipe previously indicated an equivalent amount of approximately 5 ½ cups, but I no longer recommend using cups because different brands of rolled oats may measure out differently and could result in these cookies overflowing the pan.
- Make sure you are baking your bars in a true "half sheet pan" that measures 18 x 13 x 1 inches.
- Press the dough firmly into the pan. You are aiming for a packed-down, even layer, but with the edges of the dough pushed down below the edges of the pan. This allows a little room for rising without overflowing.
Nutrition
Recipe originally shared at Yellow Bliss Road and then published on this site on January 21, 2016, and updated on March 4, 2021.
I make this recipe all the time, so great for taking treats to work! Fast to make and a reliable recipe. I use half the sugar as I find the mix-ins make them sweet enough.
I love Monster Cookies, but am a senior with arthritis, so standing long enough to make cookies is hard. Plus, I just don't have the patience anymore lol. When I found this, it looked so good! I am also gluten sensitive, so this is perfect, no flour!
I only had quick oats, and I didn't have a half sheet pan, so just used an old baking pan about the same size, but glass and deeper. I added half a cup less of sugar. I put in 1/4th cup apple sauce (my mom taught me to do that to make them stay moister lol). I added the m&ms, choc chips and also walnuts. I mixed it all together with my hands, as it is so hard to use a spoon on this thick of a mix. I baked at 350* for 45 mins. I added about half teaspoon cinnamon. They still turned out wonderful! Will definitely make them again! I love that the recipe is about half the amount of most Monster Cookie recipes and I didn't have to divide it all lol. Thank you for a great treat! They are perfect for breakfast too, with coffee. They are oatmeal after all lol.
i just made them again, still wonderful 👍 thank you so much 💟😘💟 p.s. added a touch of ginger too lol
Hi - this is my first time at your site. Your monster cookie bars look delicious. I love the large amount the receipt makes too. My question is - is there a reason that there's no flour? Are these specifically for those who can't have flour? My family does not have that issue, so I'm torn between using your recipe or one of the others that has flour.
Thank you so much for your input.
Hi Jo! These Monster Cookie Bars simply don't need flour to achieve their chewy texture. Flour was not left out of the recipe on purpose just to make them gluten-free...it's just a bonus for those who need GF. 🙂
Hi Samantha,
These bars look incredible and i am excited to make them!!!
Question: Do you need parchment paper on the sheet pan? And should i use this IN ADDITION to buttering the sheet pan? Thinking specifically of the edges
Thanks so much
Hi Anita! You should definitely generously butter your sheet pan, but the parchment paper is optional. I usually put a sheet on the bottom of the pan in addition to buttering the edges, but that's just my preference. Hope that helps and hope you enjoy! 🙂
Hi there. I recently had these at a family reunion, and they were AMAZING! I was wondering if you think the butter is necessary or if it could be replaced with more peanut butter. Then they'd be dairy free for a friend who needs it. If you don't know, I'll just give it a go. Thanks!
Absolutely the best monster cookie bar ever
I made these and they are soooo good! Thank you so much! I’ve been wanting a great bar cookie recipe for monster cookies, saves so much time for me. 👍❤️
Hi Samantha,
I love your recipes - I seem to be addicted to printing them out for my recipe binder. I would love to have the picture of the finished product print out with the recipe. Is there a way to do this that I'm missing? Thank you for helping to improve my cooking skills
Hi Carol! I saw your other comment on another post that you figured this out, Carol...at the top of the printable page, there's a little box that says "Recipe Image" which you can check.
I'm so happy to know that you're enjoying my recipes and improving your cooking skills! Take care. 🙂
Do you bake before freezing?
Yes, you would need to bake the bars before freezing. 🙂
So easy! I do not have time for making cookies. But cookie bars? I am totally on board with that! The only drawback to these bars is that they are seriously addicting. Way too yummy. There are only two of us in the house and a 1/3 of the pan has disappeared in less than 24 hours. Oh my.
Ha ha, Gayle...sounds similar to the problem we have when I make these... 😉 So happy y'all are enjoying them! 🙂
Monster cookies have been a family favorite for MANY years. Love that I can make them in a pan, now! They were our “granola bars” before they made granola bars. That’s how many years I’ve been making these!
Does this recipe not have any flour in it? And the oats don’t have to be ground up?? The pictures look great, but it doesn’t look like oats (to me). How sweet are these? Do you think I would have to use 1 less egg if I reduced the amount of sugar? Thanks!
Hi Sharon! There's no flour in this recipe, and no need to grind the oats. I would say these bars are moderately sweet as far as cookies go. I haven't tried reducing the sugar, so I'm sorry that I can't make a recommendation regarding that, but I don't think I'd use one less egg. Hope that helps and hope you enjoy!
I lowered the white sugar to only 1 cup, and added 1/4th cup applesauce and they turned out perfect ^_^
I have been making these bars for a few years now. They are quick and easy!
They freeze beautifully too! I always have some in the freezer for a back up to bring to gatherings, youth tournaments and the lake.
I'm so happy to hear this is a keeper recipe for you. 🙂 You're smart to always keep a batch in the freezer!
Hi,
I'm thinking of making these for a family reunion this weekend. With travel and meal schedule, I will need to make them 4 or 5 days before we need them. Would they be better frozen for that time? Or do you think they would still be chewy and not dried out?
Thanks!
Hi Tara! These bars stay chewy for a few days, but I do think they'd start to dry out by day 4 or 5. So I'd probably make ahead and freeze them to be safe. Enjoy your family reunion! 🙂
I used a true half sheet pan and they flooded
I don’t know if they’re cooked all the way through or not yet just took them out but they made a mess. Wish I could send pic I’m hoping they’re cooked through
I’ve made these before from a difficult site n this didn’t happen so I’m not sure what the difference is. I think this recipe is bigger
But I did use the right size pan I even measured it to double check it was. And it has sides on it too. Bummer I’m going to have to start over if they aren’t cooked all the way through
So sorry to hear that, Patti. I've made this recipe countless times -- just last weekend, in fact -- and I've never had any problem with overflow. In fact, these bars don't rise very much at all. I'm not sure what went wrong if you're sure that your pan was the correct size...all I can figure is a mistake in measuring, perhaps? Or did you use an 18-ounce tub of quick oats instead of rolled, which would have resulted in more cups of oats than the recipe calls for? At any rate, I'm very sorry that you ended up with a mess. 🙁
Don’t melt the butter, and don’t cheat on the oats.😊
I weighed my ingredients with a kitchen scale, and measured my pan before starting, and I believe my bars would have overflowed in the oven if it wasn’t for the overhang of parchment paper all around the pan. 😅 I got lucky. I wonder though, if the overflow is because I used reduced fat peanut butter. Really delicious bar still.
Is a jelly roll pan ok to use?
A standard jelly roll pan is smaller than the "half sheet pan" required for these bars, and the batter would certainly overflow. It's important to measure and make sure your pan is the correct size before you make this recipe. There's a lot of information in the post about pan size in case that helps. 🙂
Love these bars!!! So moist and chewy. I especially love not having to make them into cookies.
Me too, Bernie...so much faster and easier than baking individual cookies! I'm glad you enjoyed these. 🙂
Awesome recipe! I substituted Wow! Butter for the peanut butter so my class can make these at our nut free school and they turned out fabulous! So no gluten and no peanut butter, but an amazing bar! Thank you!!
I rarely do this, but I have to say - REALLY excellent cookie bar. I like Monster Cookies. My kids don't eat oatmeal. We are fighting over these things - hiding them from each other. The kids really liked them, and I love them. We will totally be making these again so the kids can take them to lunch at school. Thank you for a quick, easy and delicious cookie that I feel better about making for them. Actually, my son made these, so it's even less work for me!
SO happy to hear that, Crystie! 🙂
Hey Samantha...tried you Monster Cookie Bars (also added carmel M & M's) absolutely awesome!!!
Wow! First time making monster anything and I was hesitant with this recipe because it didn't have any flour. Boy was I wrong!!!! So delicious & chewy. This recipe is now my families favorite cookie recipe. Thanks for sharing.