This post may contain affiliate links. For more information, read my disclosure policy.
These Monster Cookie Bars are the ultimate kitchen sink dessert, combining everyone’s favorite flavors: oats, peanut butter, chocolate chips, and plenty of M&Ms. By turning the classic monster cookie into a thick, chewy bar, we’ve eliminated scooping and chilling. These are soft and packed with oats, perfect for school lunches, potlucks, or a late-night peanut butter craving!
Have you ever had a monster cookie? It’s a peanut butter oatmeal cookie that’s filled with chocolate chips and M&Ms. I love them, because they’re a combination of two of my favorite cookie recipes. But as much as I like cookies, I think I like cookie bars better because there is no scooping. I just press the dough in the pan and bake it, easy peasy.
This recipe starts with my favorite peanut butter cookie recipe. I adapted it to add oats to make it MONSTER. And, of course, I added chocolate chips, regular M&Ms, and PEANUT BUTTER M&M’s. I swear, peanut butter M&M’s are my downfall. I can easily eat a 1 lb. bag all by myself!
What makes this the best monster cookie bar recipe is the creamy peanut butter in the dough, semi-sweet chocolate chips, and peanut butter M&Ms. Unlike traditional monster cookies that can sometimes be dry or crumbly, our recipe includes a touch of all-purpose flour to ensure the bars hold their shape while remaining incredibly gooey in the center.
How do you make soft and chewy monster cookie bars? Beat ½ cup butter and 1 cup peanut butter with ¾ cup brown sugar and ½ cup granulated sugar until creamy. Mix in 2 eggs, vanilla, baking soda, and salt, then fold in ¾ cup flour and 3 cups quick-cooking oats. Stir in 1 cup chocolate chips and 2 cups M&Ms, press into a 9×13 pan, and bake at 350°F for 22–25 minutes until the edges are golden brown.
Ingredients Needed
- Start with unsalted butter that’s been softened. Salted butter may be substituted by reducing added salt to 1/4 tsp.
- Peanut Butter is a must for monster cookies! You can use creamy or crunchy, but use a no-stir brand (shelf-stable peanut butter, not natural). Natural peanut butter can be more oily and will change the texture of the recipe.
- Both granulated sugar and brown sugar makes a soft and chewy bar.
- This recipe was written for all-purpose flour, be sure to measure it correctly.
- For the softest, most cohesive bar, always use quick-cooking oats. Old-fashioned (rolled) oats are too large and chewy for a bar format, often causing the bars to fall apart when sliced. Quick oats act as a secondary binder, absorbing moisture and creating a dense, brownie-like texture. You can make your own quick oats from rolled oats!
- Add Chocolate Chips and M&Ms as mix ins, and I love pressing some on top for decoration. These would also be delicious if you swapped Reese’s Pieces for M&Ms (or added some in addition).
Click to see the recipe card below for full ingredients & instructions. See below the recipe card for even more information: process shots, variations, FAQ and more.
SAVE THIS RECIPE
We'll email this post to you, so you can come back to it later!
Dorothy’s Expert Tips
- When adding your M&Ms and chocolate chips, always stir them in by hand with a spatula at the very end. Using a hand mixer for the candies can crush the candy shells, streaking your beautiful batter with dye and losing that satisfying crunch in the finished bar.
- Lining the pan with foil or a parchment paper sling allows you to lift the entire block of bars out at once, making it much easier to cut perfectly straight, clean squares.
- Do not over bake these – they’re best a little under baked and gooey.
- How to serve Monster Bars: I love serving these plain, just as a bar cookie, or topping with ice cream!
Monster Cookie Bars Recipe
Recipe Video
Ingredients
- ½ cup (113 g) unsalted butter softened
- 1 cup (268 g) peanut butter
- ¾ cup (150g) packed brown sugar
- ½ cup (100g) granulated sugar
- 2 large eggs
- 1 teaspoon vanilla extract
- ½ teaspoon baking soda
- ½ teaspoon salt
- ¾ cup (93g) all-purpose flour
- 3 cups (240g) quick cooking oats
- 1 cup (170g) chocolate chips
- 2 cups M&Ms any kind, I used regular and peanut butter
Instructions
- Preheat oven to 350°F. Line a 9×13” pan with foil and spray with nonstick cooking spray.
- Beat butter and peanut butter with a hand or a stand mixer until smooth. Mix in both sugars until creamy. Add eggs, vanilla, baking soda, and salt and mix until smooth, then slowly mix in the flour and oats. Stir in the chocolate chips and M&Ms.
- Press cookie dough into the prepared pan. If desired, sprinkle more M&Ms on top for decoration. Bake for 22-25 minutes or just until the edges are golden brown. Cool before cutting into bars.
- These are great served as bars or as an ice cream sundae! Store in an airtight container for up to 3 days or freeze for up to one month.
Recipe Notes
- Make Christmas Monster Cookie Bars with holiday M&Ms
- Reese’s Pieces Monster Cookie Bars would be fantastic – just swap the M&Ms for Reese’s Pieces!
- Do not over bake these – they’re best a little under baked and gooey
- How to serve Monster Bars: I love serving these plain, just as a bar cookie, or topping with ice cream!
Recipe Nutrition
How to make Monster Cookie Bars
- Beat butter and peanut butter with a hand or a stand mixer until smooth. Mix in both sugars until creamy.
- Add eggs, vanilla, baking soda, and salt and mix until smooth.
- Slowly mix in the flour and oats. Stir in the chocolate chips and M&Ms.
- Press cookie dough into the prepared pan. If desired, sprinkle more M&Ms on top for decoration. Bake for 22-25 minutes in an oven preheated to 350°F.
Monster Cookie Bars FAQ
Bake these 22-25 minutes or until they just lose their glossy sheen.
Store these in an airtight container. Freeze for up to 3 months.
It’s not an exact swap so I suggest making gluten free monster cookies instead.
I attempted to make the recipe and was so excited to see weighted measurements! However, the weight for the oats does not match the cups measurement. I measured 2x and 256g=2 cups 🙁
What kind of oats are you using? Quaker quick oats are approximately 80g per cup.
Looks yummy! I think it shows you’re a good mom that you’re thinking about these things!
Featured In
Rate This Recipe