Monster Cookie Bars are the ultimate fusion dessert for anyone who’s ever struggled to choose between cookies, brownies, or candy. These bars combine soft and chewy peanut butter oatmeal cookie dough, melty chocolate chips, and bursts of rainbow M&M’s all in one satisfyingly thick bake. If you crave comfort and fun wrapped together in an easy-to-make treat, this is the recipe you’ll come back to time and again!

Monster Cookie Bars Recipe - Recipe Image

Ingredients You’ll Need

Despite their show-stopping looks and flavor, the ingredients for Monster Cookie Bars are simple staples you likely have on hand. Each element plays an important role in creating the perfect bite—deliciously chewy, just sweet enough, and practically overflowing with chocolate and color.

All-purpose flour: Gives the bars structure so they slice up perfectly every time.
Old fashioned oats: Add a hearty chew and a hint of nutty flavor that balances out the sweetness.
Baking soda: Provides just enough lift so your bars are thick, not dense.
Sea salt: Enhances all the flavors and counters the sweetness beautifully.
Unsalted butter (room temperature): Creates a rich, tender crumb and brings out the nutty goodness in the peanut butter.
Creamy peanut butter: The secret to soft, chewy texture and that unmistakable, craveable flavor.
Light brown sugar (packed): Adds a caramel-like depth and helps the bars stay moist.
Granulated sugar: Gives just the right amount of sweetness and helps the edges get a little crisp.
Large eggs: Bind all the ingredients together and add richness.
Vanilla extract: Rounds out the flavors with a touch of warmth.
Semi-sweet chocolate chips: Melty bursts of chocolate in every bite—pure bliss.
M&M’s (divided): For color, crunch, and those classic pops of chocolate (plus: they look so fun on top!).
Flaky sea salt (for topping): Brings out all the flavors and adds that irresistible bakery finish.

How to Make Monster Cookie Bars

Step 1: Prepare Your Pan

Start by preheating your oven to 350 degrees F. Grab a 9×13-inch baking pan and grease it generously with nonstick cooking spray. This step is crucial for easy removal and perfectly neat bars later!

Step 2: Mix Dry Ingredients

In a large bowl, whisk together the all-purpose flour, old fashioned oats, baking soda, and sea salt. Whisking helps distribute everything evenly, so your Monster Cookie Bars bake up with an even texture and flavor in every piece.

Step 3: Cream Butters and Sugars

In the bowl of a stand mixer, combine your room temperature butter, creamy peanut butter, light brown sugar, and granulated sugar. Cream them together until the mixture is smooth and fluffy, about 2 minutes. Scrape down the sides as needed—this step is essential for making sure your bars are super soft and never gritty!

Step 4: Add Eggs and Vanilla

Add the eggs and vanilla extract to the creamed mixture, and mix until everything is fully combined. You’ll notice the dough becomes gorgeously silky and fragrant at this stage.

Step 5: Combine Wet and Dry Ingredients

Gradually add the dry mixture into your wet ingredients, mixing just until combined. Don’t overmix—this keeps your Monster Cookie Bars tender instead of tough. At this stage, the dough will be thick and easy to work with.

Step 6: Fold in the Goodies

Now for the fun—stir in the semi-sweet chocolate chips and 1 cup of the M&M’s. You want these to be evenly distributed, so every bite is studded with chocolatey, candy crunch.

Step 7: Spread and Top

Scoop the dough into your prepared pan and use a spatula to spread it into an even layer. Sprinkle the remaining 1/2 cup of M&M’s over the top for a festive finish. Press them in lightly so they stay put during baking.

Step 8: Bake to Perfection

Bake for 25 to 35 minutes, keeping a close eye after 25 minutes. The bars are ready when they’re lightly golden brown and just set in the center. Don’t overbake—Monster Cookie Bars are at their best when they’re soft and chewy, not dry!

Step 9: Finish and Cool

Once out of the oven, immediately sprinkle flaky sea salt over the warm bars. This simple step makes all the flavors pop and gives a little crunch. Let the bars cool completely in the pan before cutting. Patience pays off here for clean slices!

How to Serve Monster Cookie Bars

Garnishes

A simple sprinkle of extra flaky sea salt is my favorite finishing touch, but you can also drizzle some melted chocolate or add a swirl of peanut butter on top for an even more decadent look. If you’re serving for a crowd, pop in a few extra vibrant M&M’s just before serving to make the Monster Cookie Bars look irresistibly fresh!

Side Dishes

Pair Monster Cookie Bars with a hearty glass of cold milk for the ultimate nostalgic treat, or serve them alongside vanilla ice cream for a dreamy dessert duo. They also go wonderfully with fresh berries if you want something to lighten up all the rich, chewy goodness.

Creative Ways to Present

Cut the bars into bite-size pieces for party trays, or use cookie cutters for fun shapes if you’re baking with kids. Stack them tall for bake sales, or wrap individual squares in parchment and tie them up with twine for charming homemade gifts. However you present them, Monster Cookie Bars always bring that wow-factor to the table!

Make Ahead and Storage

Storing Leftovers

Store any leftover Monster Cookie Bars in an airtight container at room temperature for up to four days. They’ll stay soft and chewy, and you can grab one whenever that sweet craving hits!

Freezing

Monster Cookie Bars are freezer-friendly! Once cooled, wrap each bar tightly in plastic wrap and pop them into a freezer bag or airtight container. They’ll keep beautifully for up to three months—just be sure to thaw them at room temperature before digging in.

Reheating

To bring your Monster Cookie Bars back to their gooey, just-baked best, microwave a square for 10–15 seconds. The chocolate chips will go all melty and the bars will taste like they’re fresh from the oven!

FAQs

Can I use quick oats instead of old fashioned oats?

Yes, you can use quick oats in a pinch, but old fashioned oats give the Monster Cookie Bars their signature chewy texture. Quick oats will make the bars a bit softer and less hearty, but still delicious!

What’s the best way to cut clean squares?

Let the bars cool completely in the pan before slicing, and use a sharp, preferably serrated knife. For super neat edges, wipe the knife clean between cuts. If you’re feeling fancy, refrigerate the bars for 20 minutes before cutting for extra-clean lines!

Can I make them gluten-free?

Absolutely! Substitute your favorite 1:1 gluten-free flour blend in place of the all-purpose flour, and make sure to use gluten-free oats. The result is just as tasty, and everyone can enjoy them!

Can I customize the mix-ins?

That’s one of the best parts! Swap the M&M’s for peanut butter chips, butterscotch chips, or even chopped pretzels. You can also add chopped nuts if you’re craving extra crunch. Monster Cookie Bars are as flexible as your imagination.

Do I need a stand mixer?

Nope! While a stand mixer makes everything easier, you can absolutely make these using a hand mixer or even good old-fashioned elbow grease and a sturdy spatula. Just be sure to cream the butter and sugars well for that soft texture.

Final Thoughts

If you’re looking for a dessert guaranteed to put smiles on faces, Monster Cookie Bars are a home run every time. Whether you’re baking for a party, a potluck, or just treating yourself, give these a try—you’ll soon see why they’ve become everyone’s favorite go-to sweet treat!