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.
Why You’ll Love This Recipe
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I can make everything in one bowl—less mess, more ease.
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The recipe doesn’t require flour, which is great for gluten-free needs.
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I like how customizable it is, from switching out mix-ins to adjusting the sweetness.
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These bars are thick, chewy, and packed with texture.
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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
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Preheat the oven to 350°F (175°C). I line a 9x13-inch baking pan with parchment paper or lightly grease it.
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In a large bowl, I cream together the butter, peanut butter, brown sugar, and granulated sugar until smooth and fluffy.
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I beat in the eggs one at a time, then add the vanilla extract and mix until everything’s combined.
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I stir in the oats, baking soda, and salt. The dough gets thick, so I mix carefully to keep the texture just right.
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I fold in the chocolate chips and most of the M&M’s, saving a handful to press on top later.
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I spread the dough evenly into the prepared pan and gently press the remaining M&M's on top.
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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
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Servings: About 24 bars
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Prep Time: 10 minutes
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Bake Time: 25–30 minutes
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Total Time: 40 minutes
Variations
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Swap the M&M’s with seasonal colors for holidays.
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Add chopped nuts for a crunchy twist.
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Try butterscotch or white chocolate chips in place of semi-sweet.
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Use almond butter or sunflower seed butter for a nut-free option.
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Mix in coconut flakes or mini marshmallows for something extra fun.
Storage/Reheating
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Room Temperature: I store them in an airtight container for up to 4 days.
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Refrigerator: They last up to a week chilled—just bring them to room temp before serving if I want them softer.
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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.
