Monster Cookie Bars are a delicious mash-up of all the best cookie ingredients—peanut butter, oats, chocolate chips, and colorful M&M's—all baked into one thick, chewy, and irresistibly sweet bar. These are perfect for parties, lunchboxes, or just a cozy night in when I want a quick, satisfying treat.

Monster Cookie Bars

Why You’ll Love This Recipe

I can make everything in one bowl—less mess, more ease.
The recipe doesn’t require flour, which is great for gluten-free needs.
I like how customizable it is, from switching out mix-ins to adjusting the sweetness.
These bars are thick, chewy, and packed with texture.
They store well and freeze beautifully for make-ahead dessert planning.

Ingredients

(Here's a tip: Check out the full list of ingredients and measurements in the recipe card below.)

unsalted butter, room temperature
creamy peanut butter
packed light brown sugar
granulated sugar
large eggs
vanilla extract
old-fashioned oats
baking soda
salt
semi-sweet chocolate chips
M&M's

Directions

Preheat the oven to 350°F (175°C). I line a 9x13-inch baking pan with parchment paper or lightly grease it.
In a large bowl, I cream together the butter, peanut butter, brown sugar, and granulated sugar until smooth and fluffy.
I beat in the eggs one at a time, then add the vanilla extract and mix until everything’s combined.
I stir in the oats, baking soda, and salt. The dough gets thick, so I mix carefully to keep the texture just right.
I fold in the chocolate chips and most of the M&M’s, saving a handful to press on top later.
I spread the dough evenly into the prepared pan and gently press the remaining M&M's on top.
Bake for 25–30 minutes, or until the edges are golden and the center is set. I let the bars cool completely before cutting.

Servings and Timing

Servings: About 24 bars
Prep Time: 10 minutes
Bake Time: 25–30 minutes
Total Time: 40 minutes

Variations

Swap the M&M’s with seasonal colors for holidays.
Add chopped nuts for a crunchy twist.
Try butterscotch or white chocolate chips in place of semi-sweet.
Use almond butter or sunflower seed butter for a nut-free option.
Mix in coconut flakes or mini marshmallows for something extra fun.

Storage/Reheating

Room Temperature: I store them in an airtight container for up to 4 days.
Refrigerator: They last up to a week chilled—just bring them to room temp before serving if I want them softer.
Freezer: I wrap individual bars in plastic and store in a freezer-safe bag or container for up to 3 months. Thaw at room temp or microwave for 10–15 seconds for a warm bite.

FAQs

How do I make the bars extra chewy?

I take them out of the oven when the edges are golden but the center still looks slightly soft. As they cool, they firm up but stay super chewy.

Can I use quick oats instead of old-fashioned?

Yes, but I prefer old-fashioned oats for that heartier texture. Quick oats make the bars a bit softer and less chewy.

What can I use instead of peanut butter?

I’ve tried almond butter and cashew butter with great results. Just make sure it’s not too runny or oily.

How do I know when they’re done baking?

When the top turns golden and a toothpick in the center comes out with just a few moist crumbs, they’re ready. I avoid overbaking to keep them soft.

Can I double the recipe?

Absolutely. I use a larger baking pan (like a sheet pan) and adjust the baking time, checking around the 30-minute mark.

Conclusion

Monster Cookie Bars are everything I love in a dessert—easy, colorful, chewy, and packed with flavor. Whether I’m baking for a crowd or just craving a homemade treat, these bars always deliver. With endless mix-in options and freezer-friendly storage, they’re one of my go-to sweet recipes.