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These Monster Cookie Bars are so full of goodies! I’m not sure which part to love more—the peanut butter flavor, the colorful M&M candies, or the ooey-gooey chocolate!

three Monster Cookie Bars stacked on top of each other

Monster Cookie Bars are basically a cookie on flavor steroids: stuffed with rolled oats, peanut butter, chocolate chips, and M&Ms.

Why are they called “monster” bars? Because they hold a monstrous amount of ingredients—but in the best way!

Lily is all over these things when we make them. It’s like all her favorite stuff in one place, haha!

The whole thing bakes in bar form, so you avoid the hassle of scooping out individual cookies. You just press the dough into a sheet pan or baking dish, slide it into the oven, and wait for the big batch of sweet squares.

And if you’re the type who loves a chewy texture, these bars deliver that “soft in the middle, crisp on the edges” vibe. Are you a corner or center piece kind of person??

Serve these as a treat at your next potluck, or keep them all to yourself at home—no judgment. I’ve been known to munch on these while I watch a late night show…

Now let’s jump into why these Monster Cookie Bars are so darn tasty. You’re gonna love ’em!

Some of our other favorite COOKIE BAR RECIPES we have on our site include: Easy Frosted Banana Bars Recipe, Fresh Peach Bars Recipe with Crumble Topping, and Peanut Butter Chocolate Chip Cookie Bars.

a Monster Cookie Bars with a bite taken out leaning against another bar

WHY THIS RECIPE WORKS:

All-in-One Convenience: Turning monster cookie dough into a bar is a great way to save time.
Chewy Meets Crispy: Using rolled oats ensures that chewy texture, while the edges become slightly crispy and golden brown.
Monstrous Flavor: Between the peanut butter, brown sugar, and m&m candies, there’s a lot going on.

INGREDIENTS NEEDED (SCROLL TO THE BOTTOM OF THIS POST FOR THE FULL RECIPE CARD):

All-purpose flour
Rolled oats
Baking powder
Salt
Butter (softened)
Peanut butter (creamy)
Brown sugar
Granulated sugar
Large eggs
Vanilla extract
M&M candies
Semi-sweet chocolate chips
Peanut butter chips (optional)

ingredients needed to make Monster Cookie Bars

Preheat the oven to 375°F. Spray a 9×13 baking dish with cooking spray and set aside.
In a medium-sized bowl stir together the flour, oats, baking powder, and salt, set aside.
In the body of a stand mixer with the paddle attachment cream together the butter and peanut butter until smooth.
Add the granulated sugar and brown sugar, stir it in until combined then place the mixer on medium-high speed and cream together for 3 minutes, scrape down the sides as needed.
Add the eggs in one at a time, fully mixing in the first before adding the next. Stir in the vanilla.
Add the dry mixture to the wet a little at a time until fully mixed in.
Stir in the M&M’s, chocolate chips, and peanut butter chips until incorporated throughout the dough.
Press the cookie dough into the prepared baking dish. Bake for 25-30 minutes until a toothpick inserted into the center comes out mostly clean. Do not overbake.
Let cool completely before cutting into bars.

collage of images showing how to make Monster Cookie Bars

FREQUENTLY ASKED QUESTIONS ABOUT THIS RECIPE:

WHAT KIND OF OATS SHOULD I USE?

Rolled oats (sometimes labeled old-fashioned) are the way to go for chewy monster cookie bars.

Quick oats can get mushy, and steel-cut would be too firm.

Rolled oats give just the right texture and help bind everything together.

CAN I USE CHUNKY PEANUT BUTTER?

Yes, go for it!

If you like little peanut pieces, natural peanut butter (chunky style) can add a nice crunch.

Just make sure you stir it well if it’s the kind that separates oil on top.

If you want a super smooth bar, stick with creamy.

a spatula holding up Monster Cookie Bars in air

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ANY ADDITIONS?

Add things to you heart’s content! This recipe can handle it.

Reese’s Pieces: Swap or add them alongside M&Ms for more peanut butter flavor.
Mini m&m: For tinier bursts of color spread more evenly throughout.
Shredded coconut: If you’re a coconut fan, stir in half a cup for extra chewiness.
White chocolate chips: Mix them in for a sweet, milky twist.
Almond butter swirl: If you adore almond flavor, swirl a bit on top of the bars before baking.
Mini marshmallows: Press them on top just before baking for gooey pockets reminiscent of a s’mores vibe.
Corn syrup drizzle: If you want an extra sticky, shiny finish, drizzle a tablespoon on top.
Chopped nuts (pecans, walnuts): A crunchy element that pairs well with peanut butter flavor.
Cocoa powder swirl: Stir a tablespoon or two into part of the dough for a two-tone effect.
Leftover halloween candy bits: Chop them up for a “surprise candy bar” feel in each bite.

ANY SUBSTITUTIONS?

I have just a few ideas here to get your brain working.

Gluten-free oats and a gluten-free flour blend if you have celiac disease or a medically restrictive diet—the end result should still be chewy and sweet.
Coconut oil in place of butter if you’re aiming for a dairy-free version—just watch that it’s solid at room temperature and measure carefully.
Flax egg in place of each egg if you’re going egg-free—whisk 1 tablespoon ground flax with 3 tablespoons water per egg, let it sit a few minutes.
Use room temperature ingredients—but if your butter is cold, you can quickly microwave it in 10-second bursts.
Dairy-free chocolate chips if you can’t do dairy—many grocery stores carry them now.
Half the sugar or try white sugar only if you prefer less sweetness, though the bars might lose some moistness or “monster” sweetness.

Monster Cookie Bars served on a white plate

HOW TO STORE:

Refrigerator
Cut the bars, place them in an airtight container lined with parchment paper, and keep them in the fridge for about a week.

They firm up a bit in cooler temps.

Let them come to room temp if you like them softer.

Freezer
Want them to last longer?

Wrap each bar individually in plastic wrap and then place them in a freezer-safe bag.

This way, you can thaw just a single piece whenever the craving strikes.

They should be good for up to 2 or 3 months.

DANA’S TIPS AND TRICKS:

Line your pan: Use parchment paper or foil with extra edges hanging over the long sides—makes lifting the entire block of bars out a breeze.
Use a large mixing bowl: It’s easier to fold in all the goodies without spillage, especially if it’s your first time.
Don’t Overbake: Remove them when the top turns golden brown but the center may still look a tad soft. They set more as they cool, preventing dryness.
Weigh your flour: If you have a kitchen scale, it can help avoid over- or under-measuring. This is a small step that goes a long way for the best texture.
Go big or small: If you want thick bars, use a smaller pan. For thinner bars and less bake time, use a larger pan like a 9×13.
Test with a toothpick: Or test the center for a bit of crumb, not raw dough. That’s how you get the best results for a chewy cookie bar that’s not overdone.

four Monster Cookie Bars stacked on top of each other, the one on top has a bite taken out

Monster Cookie Bars are the kind of cookie bar recipes that combine all your favorite things: oats, peanut butter, chocolate chips, and M&Ms.

Give them a try—you might find they’re your new favorite cookies in a whole new shape!

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If you’ve tried this MONSTER COOKIE BARS, let me know how it turned out in the comments! You can follow me over on PINTEREST, INSTAGRAM OR FACEBOOK to find some more of our mouthwatering recipes!