Monster cookie bars are peanut butter oatmeal bars filled with chocolate chips and Reese's Pieces candies. They're great for potlucks and parties, especially Halloween!
Recipe summary
Flavor/texture: Chewy dessert bars filled with peanut butter, oats, chocolate chips, and Reese's Pieces candies.
Pan size: 9x13
Yield: 24 squares
Similar to: Mini Peanut Butter M&M Cookies, Peanut Butter Pretzel Bars
Jump to:
Ingredients and substitutions
- Wet ingredients - Unsalted butter, peanut butter, granulated sugar, brown sugar, eggs, and vanilla extract add flavor and moisture to your bars. Peanut butter can be substituted with other nut and seed butters (like almond butter or sun butter) if desired.
- Dry ingredients - All-purpose flour, old fashioned oats, baking soda, and salt add structure and balance with the wet ingredients. Quick cooking oats are thin and powdery and add less texture to your bars, but can work as a substitute. Steel cut oats won't work in this recipe because they're significantly thicker and can make your bars dry.
- Mix-ins - Chocolate chips and Reese's Pieces candies could be substituted with your favorite baking chips and other candies like M&M's if desired.
How to make Monster Cookie Bars
- Stir wet ingredients in a large bowl until incorporated.
- Add sifted dry ingredients gradually and stir until just combined.
- Pour batter into a parchment lined baking pan and spread evenly.
- Bake at 350F for 26-32 minutes or until evenly browned across the top and set in the center.
Heather's Top Tip
For accurate results every time, use a kitchen scale to measure flour by weight. If you don't have a kitchen scale, use the spoon and level method. Stir the flour (especially if it's been packed down in a bag/container), then gently spoon into the measuring cup, leveling off the top with a knife. Scooping with a measuring cup compacts flour into the cup and adds up to 25% extra to the recipe, resulting in dry, bland baked goods.
Tips and tricks
Don't overmix the flour - This recipe requires only a spoon and bowl. There's no need to mix for a long period of time using a hand mixer or stand mixer. Overmixing the flour can overwork the gluten, causing your bars to turn out tough. Stir gently until you no longer see streaks of flour.
Spread batter into an even layer - Your batter will be thick, so you'll need to use a spatula to spread the batter evenly to the edges of your prepared pan. Either grease a 9x13 baking pan or line with parchment for easy removal after baking.
Pan size - You'll need a 9x13 inch baking pan for this recipe. Line your pan with parchment paper so the bars can easily be lifted from the pan once they're done baking.
Checking for doneness - Your bars should look lightly browned all across the top of the pan and look set in the center. Insert an instant-read thermometer - cookies are done at 205F. If you don't have a thermometer, insert a toothpick. It should come out with just a few moist crumbs when your bars are done.
Frequently asked questions
Yes, Reese's Pieces can be substituted with M&M's if desired. Use seasonal M&M's for Halloween or Christmas to customize your bars during the holidays!
Dry baked goods can happen for several reasons. If any of the moisture-adding ingredients have been reduced, like sugar, butter, peanut butter, or eggs, this can cause baked goods to turn out dry. If too much flour was added (weigh your flour or use the spoon and level method), baked goods can turn out dry. Lastly, baking too long in the oven can dry out baked goods. Allow your bars to cool completely before slicing and serving, because slicing into hot bars releases steam, which dries them out.
Monster cookies are a combination of several types of cookies - oatmeal cookies, peanut butter cookies, and M&M cookies. Some think that they're called monster cookies because they're craggy, bumpy, and look a bit like monsters. Others think that because it's a combination of several recipes, it's like the Frankenstein's monster of cookies.
Monster cookie bars will keep for 5-7 days in a sealed container. To help keep bars fresh, place a slice of bread in the container with the bars. Replace the bread slice as needed.
Yes, cookie bars freeze well. They'll keep for up to 3 months in a tightly sealed, freezer-safe container. To thaw, place on the countertop and remove the lid to thaw for 1 to 2 hours.
Recommended
📖 Recipe
Monster Cookie Bars
Ingredients
- ½ cup (113 g) unsalted butter, melted
- 1 cup (270 g) creamy peanut butter
- 1 cup (213 g) brown sugar
- ½ cup (99 g) granulated sugar
- 2 large eggs, room temperature
- 2 teaspoons vanilla extract
- 1 ½ cups (180 g) all-purpose flour, *measured properly
- ½ teaspoon kosher salt
- 1 teaspoon baking soda
- 3 cups (267 g) old fashioned oats
- 1 cup (170 g) chocolate chips
- 1 cup (188 g) Reese's Pieces candies
Instructions
- Preheat oven to 350℉. Line a 9x13 baking pan with parchment paper and set aside.
- In a large bowl, add melted butter, brown sugar, and granulated sugar. Stir to combine. Add eggs and vanilla extract and stir until incorporated.
- In a separate bowl, sift together the flour, salt, and baking soda. Gradually stir dry ingredients into butter mixture until just combined. Add oats, chocolate chips, and Reese's Pieces and stir until just combined.
- Pour batter into 9x13 pan and use a spoon to spread evenly to the edges.
- Bake for about 26 to 32 minutes, or until evenly browned across the top and set in the center. Use a toothpick in the center to check for doneness. Remove from oven and allow to cool before slicing and serving.
Equipment Recommendations
Notes
- * How to measure flour properly: Use a kitchen scale to measure flour for accurate results every time. If you don't have a kitchen scale, use the spoon and level method. Stir the flour (especially if it's been packed down in a bag/container), then gently spoon into the measuring cup, leveling off the top with a knife. Scooping with a measuring cup compacts flour into the cup and adds up to 25% extra to the recipe, resulting in dry, bland bars.
- Storage: Cookie bars will keep for 5-7 days in a sealed container. To help keep bars fresh, place a slice of bread in the container with the bars. Replace the bread slice as needed.
- Freezing: Cookie bars freeze well, up to 3 months in a sealed plastic container or freezer bag.
- Mix-ins: Chocolate chips and Reese's Pieces can be substituted with chopped nuts, M&M's candies, or other baking chips.
I had so much fun creating this recipe! I hope you give it a try and love it as much as I do. If you have a question, check out the post above, which is packed with information about the recipe. Tried the recipe? I'd love to hear your thoughts — leave a comment and a star rating to share your feedback!