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!
I love making monster cookie bars in the fall. This recipe makes a large batch to feed a crowd and can be ready in about an hour!
Monster cookies are known for their chewy texture, peanut butter flavor, and added mix-ins (like chocolate chips and Reese's Pieces candies). This bar form is even better because there's no chilling required and no portioning dough out onto cookie sheets.
Serve monster cookie bars any time of year for a quick and easy party dessert. They're especially great in the fall for Halloween parties and potlucks!
Ingredients and substitutions
- Unsalted butter - Can be substituted with salted butter. Reduce the amount of salt listed by ¼ teaspoon.
- Peanut butter - Adds peanut butter flavor and moisture to the bars. Peanut butter could be substituted with other nut and seed butters if desired, like almond butter or sunflower seed butter.
- Brown sugar - Adds a slightly molasses flavor to the bars as well as moisture and chewiness. Brown sugar is also slightly acidic, which reacts with the baking soda and helps your bars rise.
- Granulated sugar - Gives your cookies a crispy edge and attracts moisture, making your cookie bars moist. I do not recommend using sugar-free substitutes. Instead, search for a recipe made specifically for the sugar-free substitute you have on hand.
- Eggs - Add moisture and structure. Bring eggs to room temperature quickly by placing in a bowl of warm water. They'll be ready in minutes. Adding cold eggs to room temperature batter causes the butter to solidify, creating chunks of cold butter throughout your batter. Room temperature eggs incorporate evenly into the batter.
- Vanilla extract - Adds vanilla flavor.
- All-purpose flour - Adds structure and balances with the wet ingredients. I have not tested this recipe with other types of flour. Other flours (like almond flour or coconut flour) cannot simply be substituted at 1:1 ratio. If you would like to bake using other flours, I highly suggest searching for a recipe that has been written specifically for those ingredients.
- Oats - Add structure and chewy texture to the bars. I recommend using old fashioned oats for best results. Quick cooking oats are thin and powdery and add less texture to your bars, but can work as a substitute. I don't recommend using steel cut oats because they're significantly thicker and can make your bars dry.
- Baking soda - Works with the acidic ingredients and helps your bars rise in the oven. Do not substitute with baking powder - these two ingredients are different and cannot be substituted for one another.
- Salt - Salt enhances the flavor of your other ingredients and balances with the sweetness. If using salted butter, reduce the amount of salt listed by ¼ teaspoon.
- Chocolate chips - I recommend semi-sweet chocolate chips because they don't add too much sweetness. These can be substituted with any flavor baking chips (like white chocolate chips or peanut butter chips) if desired.
- Reese's Pieces candies - I like to use Reese's Pieces candies in my monster bars because they go well with the peanut butter flavor in the base and they've got the perfect colors for fall and Halloween! Feel free to use M&M's candies or substitute with more chocolate chips if desired.
Tips and tricks
Properly measure your flour - Too much flour can cause your bars to turn out thick and dry. Either use a kitchen scale to weigh your flour (this is what I do and recommend), or spoon the flour into your measuring cup and level off with a knife. Scooping with the measuring cup directly from a bin of flour compacts it into the cup, adding up to 25% extra flour to the recipe. Check out my flour measuring test on my post 10 tips for baking cookies to see the weight difference of each measuring method!
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. I recommend lining 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. Insert a toothpick into the center. If it comes out with wet batter, your bars need more time. If it comes out with crumbs or nothing at all, your bars are ready to come out of the oven.
Storage
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.
Cookie bars also 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.
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 (I recommend weighing your flour or using 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.
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!