Monster Cookie Bars – Thick, Chewy, and Loaded with Goodies

Author: Sharis Mariner

Published: December 7, 2025

lina033321 close up of thick monster cookie bars on a clear g 572b9ac3 d30b 46c0 94a8 95978c072ba2 1 - Monster Cookie Bars – Thick, Chewy, and Loaded with Goodies

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Ingredients You’ll Need

Here’s your shopping (or pantry) list for these Monster Cookie Bars.

Wet Ingredients

1/2 cup unsalted butter, softened
1 cup brown sugar, packed
1/2 cup granulated sugar
1 cup creamy peanut butter
2 large eggs
2 teaspoons vanilla extract

Dry Ingredients

1 1/4 cups all-purpose flour
1 cup old-fashioned rolled oats
1 teaspoon baking soda
1/2 teaspoon salt

Mix-Ins

3/4 cup semi-sweet chocolate chips
1 cup M&M candies (plus a little extra for sprinkling on top, if you like)

Optional Add-Ons

1/2 cup chopped peanuts, pecans, or walnuts
Flaky sea salt for sprinkling
Extra chocolate chips for the top

Tools You’ll Need

You don’t need anything fancy for Monster Cookie Bars, which is exactly why I love them.

9×13-inch baking pan
Parchment paper (optional but helpful)
Mixing bowls (one large, one medium)
Hand mixer or stand mixer (or a strong arm and a wooden spoon)
Measuring cups and spoons
Rubber spatula or wooden spoon
Cooling rack

How to Make Monster Cookie Bars

These Monster Cookie Bars come together quickly, and the batter is honestly good enough that you’ll want to lick the spoon.

Prep the pan and oven
First, preheat your oven to 350°F (175°C).
Then lightly grease a 9×13-inch pan and, if you want easy cleanup, line it with parchment paper, leaving some overhang to lift the bars out later.
Cream the butter, peanut butter, and sugars
In a large mixing bowl, add the softened butter, creamy peanut butter, brown sugar, and granulated sugar.
Beat everything together on medium speed until the mixture looks light and fluffy, about 2–3 minutes.
Add the eggs and vanilla
Next, beat in the eggs one at a time until fully combined.
Then add the vanilla and mix again. The mixture should look smooth and creamy, and it already smells amazing.
Whisk the dry ingredients
In a separate medium bowl, whisk together the flour, oats, baking soda, and salt.
Make sure the baking soda and salt distribute evenly so your Monster Cookie Bars bake consistently.
Combine wet and dry
Now, add the dry mixture to the wet mixture in two batches.
Mix on low speed just until no streaks of flour remain. Don’t overmix here, because you want thick, chewy Monster Cookie Bars, not tough ones.
Fold in the fun stuff
Gently fold in the chocolate chips and M&Ms with a spatula.
If you’re using nuts or extra candy, stir those in now too. The dough will be thick and chunky in the best possible way.
Press and top
Spread the dough evenly into your prepared pan.
Then use the spatula or your fingertips to press it into an even layer. For extra “wow,” sprinkle a few more M&Ms and chocolate chips over the top and press them in lightly.
Bake to chewy perfection
Bake the Monster Cookie Bars for 22–28 minutes, depending on your oven.
When the edges look golden and the center looks set but slightly soft, you’re good. The bars will firm up as they cool, so don’t overbake.
Cool and slice
Let the bars cool in the pan on a rack for at least 20–30 minutes.
Then lift them out using the parchment (if you used it) and cut into squares or rectangles.

I still remember the first time I baked Monster Cookie Bars for a bake sale at my kids’ school. I was running late, the kitchen looked like a candy store exploded, and I basically dumped everything fun into one pan. Then I pulled them out of the oven, saw those melted chocolate chips and colorful M&Ms, and thought, “Okay… these might actually be dangerous.”

Everyone kept coming back “just for one more piece,” and the pan vanished in under fifteen minutes. Now, whenever I need an easy, crowd-pleasing dessert, I reach for this Monster Cookie Bars recipe. It’s got the cozy vibes of an oatmeal cookie, the peanut butter richness of a classic PB cookie, and the candy-studded joy of a kids’ birthday party.

What Makes This Recipe Special

Thick, chewy texture: The combo of oats, peanut butter, and butter creates soft, chewy Monster Cookie Bars that hold together but never feel dry.
Loaded with mix-ins: Every bite has chocolate chips and M&Ms, so each square looks bakery-worthy without any decorating skills.
Perfect for parties and potlucks: You bake once, slice into bars, and feed a crowd without scooping individual cookies.
Easy to customize: You can swap candies, change the chocolate, or add nuts and still keep the same base Monster Cookie Bars recipe.

lina033321 close up of thick monster cookie bars on a clear g 572b9ac3 d30b 46c0 94a8 95978c072ba2 0 - Monster Cookie Bars – Thick, Chewy, and Loaded with Goodies

Quick Ingredient Swap Table

You can easily tweak these Monster Cookie Bars based on what’s in your pantry.

Ingredient	Swap Option	Flavor/Texture Change
Creamy peanut butter	Almond or cashew butter	Milder nut flavor, still rich.
Old-fashioned oats	Quick oats	Slightly softer, less chewy bars.
Semi-sweet chocolate chips	Milk or dark chocolate chips	Sweeter or deeper chocolate profile.
M&M candies	Reese’s Pieces or chopped candy bars	Extra peanutty or candy-bar pockets.
All-purpose flour	1:1 gluten-free baking blend	Gluten-free Monster Cookie Bars option.

What to Serve With Monster Cookie Bars

Monster Cookie Bars already taste like a party, but you can turn them into a full dessert spread.

Pair them with cozy fall treats like Irresistible Cranberry Bars and let everyone choose their favorite bar. Best Homemade Recipes
Serve them alongside other bar desserts such as Oatmeal Breakfast Bars: Perfection for an oat-lover’s dessert board. Best Homemade Recipes
Create a seasonal dessert tray with Monster Cookie Bars, Pumpkin Bars: The Cozy Fall Treat You NEED and Zucchini Bars: Deliciously Sneaky Veggie Dessert so everyone gets a little bite of everything. Best Homemade Recipes+1

Add cold milk, hot coffee, or a scoop of vanilla ice cream, and you’ve basically built dessert heaven.

Tips for Perfect Results

Don’t overbake
Because these Monster Cookie Bars should stay chewy, pull them out when the center still looks slightly soft. They keep cooking a bit in the hot pan.
Press the dough evenly
For even baking, spread the dough all the way into the corners and level the top. Uneven dough leads to underbaked centers and crunchy edges.
Save some candy for the top
If you scatter extra M&Ms and chips on top before baking, your Monster Cookie Bars will look bakery-level beautiful without any frosting.

Storage Instructions

Monster Cookie Bars store like a dream, which makes them perfect for make-ahead treats.

Room temperature: Store cut bars in an airtight container at room temp for up to 3–4 days.
Fridge: For extra longevity, keep them in the fridge for up to a week. Let them come to room temperature before serving for the best texture.
Freezer: Wrap individual bars in plastic wrap, then place them in a freezer bag. Freeze for up to 2–3 months. Thaw at room temp or microwave in 10–15 second bursts.

FAQ – Monster Cookie Bars

1. Do Monster Cookie Bars need to be refrigerated?

You don’t have to refrigerate Monster Cookie Bars right away. You can keep them in an airtight container on the counter for a few days. However, if you want them to last longer and stay extra chewy, you can refrigerate them and bring them to room temp before serving.

2. Can I make Monster Cookie Bars without oats?

Yes, you can make Monster Cookie Bars without oats by swapping the oats for more flour. The bars will taste more like classic peanut butter chocolate chip cookie bars and feel slightly less chewy, but the recipe still works.

3. How do I know when Monster Cookie Bars are done baking?

You’ll know the Monster Cookie Bars are done when the edges turn golden brown and the center looks set but still soft. If a toothpick in the center comes out with moist crumbs (not wet batter), they’re ready.

4. Can I make Monster Cookie Bars gluten-free?

Yes. You can use certified gluten-free oats and replace the all-purpose flour with a 1:1 gluten-free baking blend. The texture of your Monster Cookie Bars may be slightly different, but they still taste amazing.

5. Can I double this Monster Cookie Bars recipe?

Absolutely. You can double the recipe and bake it in two 9×13 pans or use a larger, deeper pan. Just watch the baking time and add a few extra minutes if the bars are thicker.

6. How do I keep Monster Cookie Bars soft the next day?

To keep Monster Cookie Bars soft, store them in an airtight container with a small piece of bread or a marshmallow. The bars absorb a bit of the moisture and stay chewy instead of drying out.

Conclusion

Monster Cookie Bars are the dessert you make once and then keep in your permanent rotation. They come together quickly, they use simple pantry ingredients, and they bring that thick, chewy, candy-studded joy every single time. Plus, they travel well, slice beautifully, and disappear from the plate faster than you can say “save me a piece.”

So go grab your butter, brown sugar, peanut butter, and a bag of M&Ms. Then bake a pan of Monster Cookie Bars, slice them into generous squares, and share them with your favorite people. Don’t forget to snap a picture, post it on TikTok or Instagram, and tag us so we can see your creation. And if you love easy, fun desserts like this, be sure to leave a comment and hop on the newsletter list so you never miss the next viral-style treat.