Oatmeal Monster Brownies

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Oatmeal Monster Brownies are made with a layer of chewy oatmeal cookies with M&Ms, peanut butter chips, and semisweet chips, on top of fudgy brownies.

This easy and fun Brownie Recipe is based around delicious Monster Cookies. You get all the same fun mix-in ingredients found in the cookies, but with an additional layer of rich, chocolatey brownies.

Oatmeal Monster Brownies in stack


There’s nothing like a batch of brownies fresh out of the oven for the perfect family-size dessert! This version takes Classic Brownies and adds a fun layer of monster cookie dough over the top. So, you get all the best flavors from chewy monster cookies and fudgy brownies.

Oatmeal Monster Brownies prep steps, batter in baking dish, bowl of oatmeal topping, and topping being applied to brownies

If you haven’t had them before, monster cookies are like a mashup of a few popular cookie recipes. With a chewy oatmeal base, chocolate chips, M&Ms, and peanut butter flavor, you get a little bit of everything, and they’re an instant family favorite!

Oatmeal Monster Brownies in baking dish

If you’ve been following along for a while you probably remember Monster Brownies, which takes the mix-ins from our favorite monster cookie recipe and simply adds them to brownies. The difference with Monster Cookie Brownies is that we’ve also added the oatmeal cookie dough and not just the mix-ins. Every bite is like a combination of a cookie and a brownie for the ultimate sweet treat!



  • Prep time: Start the dessert recipe off by preheating your oven and spraying a 9×13 inch baking dish with non-stick cooking spray, so that they’re ready to go when the dough is done. 
  • Oatmeal Monster Cookie Dough: Add the old-fashioned oats, brown sugar, softened butter, flour, and baking soda to a large mixing bowl. Whisk the dough together until just combined. Then fold in the chocolate chip, peanut butter chip, and M&M mix-ins. 
  • Brownie layer: Add water to a medium saucepan and bring it to a boil. Turn off the heat before you add in the butter and cocoa powder. Whisk until the chocolate mixture is well blended, then pour it into a large mixing bowl. Mix in the granulated sugar, eggs, and vanilla extract until the batter is creamy. Sift in the dry ingredients and mix until just combined, but be careful not to overmix the batter. 
  • Baking time: Pour the brownie batter into the prepared baking dish and use an offset spatula to spread it out evenly. Then spread the oatmeal cookie dough over the top. Bake for 35-40 minutes until the brownies are set and the cookie dough is a beautiful golden brown color. Let the bars cool completely before slicing into squares.

Oatmeal Monster Brownies on cutting board

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  • Chocolate chips: You can use different chocolate chips besides semi-sweet chocolate chips depending on your preference. Try using milk chocolate, dark chocolate, or white chocolate chips to change up the chocolatey flavor. 
  • Mix-ins: There are plenty of other delicious mix-ins you can add to the cookie layer. Reese’s Pieces, peanut M&Ms, toffee bits, pretzel pieces, or chopped nuts would all make tasty and fun additions.
  • Peanut Butter Brownies: For a fun twist on this recipe, you can make Peanut Butter Brownies for the brownie layer in the recipe. 



  • Serve: After baking the Monster Cookie Bars, they’ll be good at room temperature for 1-2 days. Cover baking pan in plastic wrap or foil to keep them moist. 
  • Store: You can also cover them or put them in an airtight container to store in the fridge for up to 1 week. 
  • Freeze: To freeze the brownies, slice them into squares and put them in an airtight container with the layers separated by parchment paper. They should stay good frozen for up to 3 months.

Oatmeal Monster Brownies on cutting board

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Oatmeal Monster Brownies

  • Yield: 18 Servings
  • Prep Time: 10 minutes
  • Cook Time: 35 minutes
  • Total Time: 45 minutes
  • Course: Dessert
  • Cuisine: American
  • Author: Sabrina Snyder

Oatmeal Monster Brownies are made with a layer of chewy oatmeal cookies with M&Ms, peanut butter chips, and semisweet chips, on top of fudgy brownies.


Oatmeal Monster Cookie Layer:

  • 1 1/2 cups quick-cooking oats
  • 1/2 cup brown sugar , packed
  • 1/2 cup butter , melted
  • 1/2 cup flour
  • 1/2 teaspoon baking soda
  • 1/2 cup semi-sweet chocolate chips
  • 1/2 cup peanut butter chips
  • 1/2 cup M&M candies

Brownie Layer:

  • 1/2 cup water
  • 2/3 cup unsalted butter
  • 3/4 cup unsweetened cocoa powder
  • 2 cups sugar
  • 2 large eggs
  • 1 teaspoon vanilla extract
  • 1 1/2 cups flour
  • 1/2 teaspoon baking soda
  • 1/4 teaspoon salt


Note: click on times in the instructions to start a kitchen timer while cooking.

Oatmeal Monster Cookie Layer:

  1. In a large bowl whisk together oats, brown sugar, butter, flour, and baking soda until well combined.
  2. Stir in chocolate chips, peanut butter chips, and M&M candies.

Brownie Layer:

  1. Preheat oven to 350 degrees and spray a 9x13 baking dish with baking spray.
  2. To a saucepan bring water to a boil.
  3. Turn off the heat, add in the butter and cocoa powder.
  4. Whisk until completely blended.
  5. Pour mixture into a large mixing bowl.
  6. Whisk in the sugar, eggs, and vanilla extract until creamy.
  7. Add flour, baking soda, and salt until just combined.
  8. Pour into the baking dish and spread evenly.
  9. Spread Oatmeal chip dough over the brownie batter evenly.
  10. Bake for 35-40 minutes.

  11. Cool completely before slicing.

Nutrition Information

Yield: 18 Servings, Amount per serving: 390 calories, Calories: 390g, Carbohydrates: 55g, Protein: 5g, Fat: 18g, Saturated Fat: 11g, Trans Fat: 1g, Cholesterol: 54mg, Sodium: 164mg, Potassium: 142mg, Fiber: 3g, Sugar: 36g, Vitamin A: 413g, Vitamin C: 1g, Calcium: 32g, Iron: 2g

All images and text © for Dinner, then Dessert.

Keyword: Oatmeal Monster Brownies

Oatmeal Monster Brownies collage

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