Monster Cookie Bars Recipe: Oats, Peanut Butter & M&Ms

These flourless monster cookie bars combine quick-cooking oats, all‑natural peanut butter, M&M’s, and chocolate chips into a chewy, crowd-pleasing dessert. They bake in a single 9×13 pan and slice easily for gatherings, lunchboxes, or a quick sweet snack. Enjoy!

What are Monster Cookie Bars?

Monster cookie bars are essentially a pan-sized version of classic monster cookies: all the familiar flavors of oats, peanut butter, M&M’s, and chocolate chips pressed into a single sheet and baked until chewy. Because they’re made without flour, these bars have a dense, satisfying texture and can be a naturally gluten-free treat when you use gluten-free oats.

They’re perfect when you want the cookie experience without scooping dozens of rounds—prepare one batch, slice into bars, and you’re ready to go.

Why you’ll love them!

• Flourless and naturally hearty: Quick-cooking oats replace flour for a chewy, gluten-free base.
• Simple, real ingredients: The recipe highlights all‑natural peanut butter (just peanuts) and pantry staples like maple syrup and brown sugar.
• No granulated sugar needed: Light brown sugar adds sweetness and rich flavor; you can substitute coconut sugar 1:1 if you prefer.
• Chocolatey mix-ins: Semi-sweet or dark chocolate chips create depth, while M&M’s add colorful crunch—classic monster cookie vibes.

Other tasty mix-in options to try instead of or in addition to M&M’s:

• Butterscotch chips
• Peanut butter chips
• White chocolate chips
• Raisins or chopped dried fruit

What You Need for Monster Bars

This recipe uses just nine ingredients most home bakers keep on hand. Below are the exact amounts and a few suggestions to help you adapt the bars to your pantry.

• 4½ cups quick-cooking oats (use gluten-free oats if needed; quick-cooking oats are preferred—rolled oats tend to absorb too much liquid and can make the bars dry)
• 1 cup all‑natural drippy peanut butter (just peanuts—no added oils or sugars)
• ½ cup maple syrup
• ½ cup butter, melted (we recommend using butter for best texture)
• 2 large eggs
• 1 cup light brown sugar (or substitute coconut sugar 1:1)
• 2 teaspoons baking soda
• 1 cup M&M’s
• ½ cup semi‑sweet chocolate chips

How to Make Monster Cookie Bars

Note: this is a one-bowl recipe. The full ingredient list with measurements appears above.

Preheat & Prep

Preheat the oven to 350°F (175°C). Lightly spray or line a 9×13-inch baking dish with cooking spray or parchment paper for easy removal.

Mix Ingredients

In a large bowl combine the oats, peanut butter, maple syrup, melted butter, eggs, brown sugar, and baking soda. Stir with a wooden spoon until well combined—the mixture should be thick and similar to cookie dough.

Fold in the M&M’s and chocolate chips last so they don’t melt from the warm butter.

Transfer Dough to Baking Dish

Press the dough firmly and evenly into the prepared 9×13 pan using a wooden spoon or lightly greased hands so the top is compacted and flat.

Bake

Bake at 350°F for 17–20 minutes, or until the edges begin to turn golden brown and the center feels set but still slightly soft. Avoid overbaking to keep the bars chewy.

Let Cool & Enjoy!

Allow the monster cookie bars to cool in the pan for at least 30 minutes to 1 hour before slicing. Cooling fully makes slicing cleaner and improves texture. Serve room temperature or slightly warm.

Storage

Once completely cooled, store bars in an airtight container at room temperature for up to 5 days. If your baking pan has a tight-fitting lid you can keep them stored right in the pan.

Can I freeze monster cookie bars?

Yes. Two easy freezing options:

Dough: Wrap the unbaked dough tightly in plastic wrap and place it in a freezer-safe bag for up to 3 months. Thaw before baking.

Baked bars: Cool completely, cut into individual bars, and freeze in a freezer-safe bag for up to 3 months. Thaw at room temperature before serving.

Nutrition

Approximate nutrition per serving (1 of 16): Calories: 389 kcal; Carbohydrates: 49 g; Protein: 8 g; Fat: 20 g; Fiber: 5 g; Sugar: 18 g. Nutrition information is an estimate and should be used as a guideline only.