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!

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.

1. Mix together the eggs and the sugars.
   
2. Beat in the salt, vanilla, peanut butter, and butter.
   
3. Stir in the baking soda and the oats.
   
4. Mix in the goodies!
   
5. 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.
   
6. 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.
   
7. 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!

1. 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.
2. 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.
3. 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. 😉

More Tasty Bars

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• Pecan Pie Bars
• Lemon Sugar Cookie Bars
• Oatmeal Peanut Butter Energy Bars

Recipe originally shared at Yellow Bliss Road and then published on this site on January 21, 2016, and updated on March 4, 2021.