These monster cookie bars are impossible to resist and loaded with chocolate chips, peanut butter, and M&Ms. Made in just one bowl, they’re a huge hit with kids and ready in about 30 minutes.
Easy monster cookie bars are my go-to dessert when I need to bake for a crowd/potlucks, or send in a school treat, especially if it’s at the last minute! Kids and adults both love them thanks to their soft, chewy texture, M&M’s, and peanut butter flavor.
This large-batch recipe yields 4 dozen bars and takes only 30 minutes. Plus, they come together in one bowl, which makes cleanup quick and simple. And all you need is a few pantry staples! If you love a dessert bar, try my caramel chocolate chip cookie bars or my lemon raspberry cheesecake bars.
Table of Contents
Why you’ll love this recipe
Think monster cookies, but in a bar form!
- Fun for kids and easy to serve. They love the bright and colorful M&Ms. It’s also the easiest treat to transport to birthday parties, soccer games, and school events, and pretty mess-free.
- Customize the recipe. Feel free to play around with the classic recipe; use holiday-themed M&Ms, add nuts, or peanut butter chips!
- Make the bars gluten-free. This recipe doesn’t call for any flour. Use gluten-free oats, and you’re set!
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Pin ItKey Ingredient Notes
- Unsalted butter adds rich flavor and moisture to the dough, especially when paired with rolled oats. The oats will soak up the butter, making them nice and soft.
- Brown sugar and granulated sugar – When used together, these add sweetness to the bars while keeping them moist. Use light or dark brown sugar, whatever you prefer.
- Eggs – Room-temperature eggs are ideal, because they blend more seamlessly into the dough, improving the overall texture.
- Creamy peanut butter – Smooth (not crunchy) peanut butter is best. Peanut butter isn’t added just for flavor; it’s a crucial binding ingredient contributing to the soft and chewy texture! Try to avoid natural peanut butter as it tends to separate and can cause your bars to be runny.
- Old-fashioned rolled oats – These give the bars structure, holding the ingredients together while enhancing the texture. No quick oats or oatmeal here.
- Baking soda – This helps the cookie bars rise. Make sure it’s fresh!
- Chocolate chips – I recommend using semi-sweet chocolate chips. Since the dough contains added sugar and M&Ms, a sweeter chocolate chip, like milk chocolate, isn’t necessary.
- M&Ms – These add color and sweetness and are just plain fun! I prefer regular-sized M&Ms, but mini M&Ms also work.
How to make monster cookie bars
Before you get started, preheat your oven to 350°F, and line a baker’s half baking sheet with parchment paper.
1. Make the monster cookie dough
- Cream the butter, brown sugar, and granulated sugar on medium-high speed in an electric mixing bowl fitted with the paddle attachment. Beat until nice and fluffy, about 3 minutes.
- Add the eggs, peanut butter, and vanilla extract, and continue mixing on medium-high speed until well combined, about 2 minutes.
- Add the dry ingredients (rolled oats and baking soda), and continue mixing for about 1 more minute.
- Stir in the add-ins, like chocolate chips and M&Ms.
2. Prepare the monster cookie bars
- Transfer the monster cookie dough to your prepared cookie sheet.
- Spread the mixture out into an even layer on the baking sheet using a silicone spatula.
3. Bake and cut the bars
- Bake the bars for 15 minutes or until they’re golden brown and just set in the middle.
- Don’t overbake! They’ll continue to set and firm up as they cool.
- Let the bars cool completely before slicing them into 48 equal-sized squares.
Decorate
Make these M&M cookie bars extra fancy by top them with royal icing or powdered sugar icing, and add your favorite toppings!
Expert Tips
- Make thicker bars using a smaller baking pan. If you’d prefer extra-thick cookie bars, use a 9×13-inch baking pan instead of a half baking sheet. Keep in mind that a smaller pan will result in fewer bars and require a longer baking time of 18 minutes instead of the usual 15 minutes.
- Leave some overhang when lining the baking sheet with parchment paper. Extra parchment around the edges will give you handles to grab onto, making the bars so much easier to remove from the pan!
- Spread the dough out into an even thickness. An even layer of dough ensures the bars cook consistently throughout. Use a silicone spatula or your hands to press the dough firmly into the pan.
- Use regular creamy peanut butter. Avoid using natural or crunchy since it will change the texture of the cookie dough before baking.
- Don’t overbake! Overbaking can result in dry, crumbly cookie bars that fall apart. Trust me, pull the bars out of the oven right at 15 minutes (or 18 minutes for a smaller pan). They’ll finish baking as they cool.
- Plan for at least 1 hour of cooling time. For hand-held cookie bars that don’t fall apart, it’s essential to wait until they’re completely cooled before cutting them into squares.
Variations
When it comes to making these monster cookie bars your own, the options are endless. Here are some other ideas for mix-ins.
- Chopped nuts like peanuts or walnuts
- Swap the peanut butter for almond, cashew, or sunflower butter
- Make them festive for any holiday with colored M&Ms
- Try butterscotch or white chocolate chips
- Stir in a dried fruit like cherries or cranberries
- Drizzle with icing, melted chocolate, or even a caramel glaze
Storage
These are a great treat to keep around the house during the week. Store cooled monster cookie bars in an airtight container on the counter for up to 1 week. I’m sure you’ll have them eaten up long before that!
Cookie bars are freezer-friendly too! Wrap each cookie bar tightly in plastic wrap to prevent freezer burn. Then, place the bars in a plastic ziplock freezer bag for extra protection. Freeze for up to 3 months.
Let frozen bars thaw at room temperature before serving.
To Make-Ahead:
If you’re trying to minimize cleanup on baking day, consider preparing your dough in advance.
- Prepare the cookie dough as directed in the recipe.
- Cover the prepared dough tightly in plastic wrap, and store it in the fridge for 2-3 days.
- Let the cookie dough sit at room temperature for at least 30 minutes before spreading it in the baking pan. This will soften the dough a bit, making it much easier to work with.
- Follow the remaining recipe steps when you’re ready to bake!
FAQs
Most cookie bars are baked on a baker’s half baking sheet, measuring 17.9 x 12.9 x 1 inches. A baking sheet with a rim is necessary because the cookie bars will rise. Alternatively, a 9×13-inch baking dish also works. You’ll just end up with thicker bars that need a few extra minutes in the oven.
Besides M&Ms and chocolate chips, you’ll find monster cookie bars containing pretzel pieces, walnuts, peanut butter cups, and even leftover Halloween candy! Feel free to have fun with your mix-ins!
When the bars are fresh out of the oven, they may appear gooey, especially towards the middle of the pan. A soft-looking center is normal and doesn’t mean a longer baking time is needed. They’ll firm up as they cool down.
Yes, feel free to swap out the peanut butter with any other nut or seed butter such as almond butter, cashew butter, or sunflower butter to cater to different allergies. The flavor will change slightly, depending on what you use, but the bars will still be tasty!
More Dessert Bar Recipes:
Monster Cookie Bars
Ingredients
- 1 stick (113 g) unsalted butter at room temperature
- ¾ cup (150 g) brown sugar Note 1
- ¾ cup (150 g) granulated sugar
- 3 large eggs at room temperature
- 1 ½ cups (400 g) creamy peanut butter
- 1 teaspoon pure vanilla extract
- 4 ½ cups (400 g) old-fashioned rolled oats
- 2 teaspoons baking soda
- ¾ cup (120 g) semi-sweet chocolate chips
- 1 cup (200 g) M&Ms
Equipment
- stand mixer fitted with a paddle attachment
Instructions
- Preheat the oven to 350°F (177°C). Line a baker’s half baking sheet with a parchment paper. (You may use 9×13 baking pan as well for thicker bars.)
- In a mixing bowl with paddle attachment, cream butter, brown and granulated sugars on medium high speed until nice and fluffy, about 3 minutes.1 stick unsalted butter, ¾ cup brown sugar, ¾ cup granulated sugar
- Add eggs, peanut butter, and vanilla extract. Mix on medium high speed until well combined, about 2 minutes.3 large eggs, 1 ½ cups creamy peanut butter, 1 teaspoon pure vanilla extract
- Add rolled oats and baking soda and mix until well combined, another minute or so.4 ½ cups old-fashioned rolled oats, 2 teaspoons baking soda
- Stir in chocolate chips and M&Ms.¾ cup semi-sweet chocolate chips, 1 cup M&Ms
- Spread the mixture into an even layer in the prepared baking sheet.
- Bake for 15 minutes (18 minutes for 9×13 baking pan). Don’t overbake! Cool completely.
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Tips & Notes
– Store leftover monster cookie bars in an airtight container at room temperature for up to 1 week.
– Freeze your monster cookie bars for up to 3 months. Wrap each bar tightly in plastic wrap, and place them in a ziplock freezer bag.
– Thaw frozen bars at room temperature before serving.
Nutrition
