Monster Cookie Bars are a holiday favorite and these delicious chewy bars are made with old-fashioned oats, peanut butter, chocolate chips, and M&M’s.

These quick and easy-to-make bars are made with simple everyday ingredients are a fun addition to cookie tins during the holidays!

Ingredients in Monster Cookie Bars

• Unsalted butter
• Brown sugar
• Granulated sugar
• Egg
• Salt
• Vanilla extract
• Peanut butter - I prefer creamy in my monster cookies!
• All-purpose flour
• Baking soda
• Old fashioned oats
• Semisweet chocolate chips
• M&M’s

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How to Make Monster Cookie Bars

Preheat the oven and prepare the baking dish - Preheat the oven to 350°F and line a 9x9 baking dish with parchment paper and lightly grease the sides of the pan.

TIP: Allow the parchment paper to overhang on two sides so that once the monster cookie bars are done it is easy to lift the bars out of the dish without sticking.

Blend the wet ingredients- Using a hand mixer or stand mixer cream together the butter, granulated sugar, and brown sugar. Add in the egg and vanilla, and blend until well combined. Next, the peanut butter is beat into the mix until the mixture is smooth.

Mix the dry ingredients- Combine the flour and the baking soda together and then mix it into the peanut butter mixture. Blend in the old-fashioned oats and stir until the oats are moistened.

Add the mix-ins- Stir in M&M’s and the chocolate chips until the mix-ins are mixed into the dough.

RELATED: Chocolate Chip Cookie Bars

Transfer the dough to the baking dish- Evenly spread the mixture into the prepared 9x9 baking dish. Sprinkle the top with additional M&M’s and chocolate chips then gently press them down into the top of the dough.

Bake the monster cookie bars - Bake for 18-20 minutes. The outer parts of the dish should be lightly brown but the center will not quite be done. The bars will continue to bake while cooling in the pan.

Allow the dish to cool before slicing-Allow to cool completely before cutting into squares and removing it from the pan to prevent crumbling. This will allow the bars to “set” so they will be tender and chewy.

Gluten Free Monster Cookies

This recipe has flour in it so it's not gluten-free. However, my recipe for Monster Cookies is gluten-free and can even be made nut-free! The monster cookies version is a family tradition that I made as a child with my mother and I now make with my kids! If you're looking for a peanut butter alternative, Wowbutter is a great option and it provides the same creaminess and delicious flavor as peanut butter in this recipe.