In a large bowl, whisk together the sugar, flour, cocoa powder, baking powder, baking soda, and salt until very well combined. Transfer to a gallon-sized plastic baggie. (Repeat for as many cake mixes as you wish to assemble.)
TO USE CAKE MIX:
Adjust oven rack to the center position and preheat oven to 350°F. Grease and flour a (9- by 13-inch) rectangular baking pan, OR two (9-inch) round baking pans, OR line 24 muffins cups with paper baking liners.
Pour the Homemade Chocolate Cake Mix into a large bowl. Blend in the eggs, milk, oil, and vanilla using an electric mixer. Beat on medium speed for 2 minutes, scraping the bowl as needed. Gradually stir in the boiling water (¼ cup at a time) until the batter is well-combined (it will be thin and runny).
Pour the batter into the prepared pans, dividing evenly. Bake until a wooden toothpick inserted in the center comes out clean, using these times as a guideline: one (9- by 13-inch) rectangular baking pan = 35 to 40 minutes; two (9-inch) round baking pans = 30 to 35 minutes; 24 cupcakes = 20 to 22 minutes.
Cool cake(s) in pans for 10 minutes (only 5 minutes for cupcakes) before turning out onto a wire rack. Allow to cool completely before frosting.
Video
Notes
I prefer whisking the dry ingredients together in a bowl, transferring the resulting mix to a baggie, and then repeating the steps in the same bowl for each additional cake mix I wish to assemble.
I like to mix up three Homemade Chocolate Cake Mixes at a time with the intent to use them up within six months. Just be sure to write the date on the bags so you remember how old they are.
Always use fresh baking powder and baking soda when assembling homemade baking mixes to ensure that they will keep fresh in the pantry longer.
After you pour the boiling water into the batter, it will seem unusually thin. But don't be alarmed! This method produces a light, moist, and tender cake.
Nutrition values below are calculated for one serving of chocolate cake (not just one serving of cake mix), when made as directed using all ingredients.