Monster Cookie Bars

Indulge in gooey goodness with our Monster Cookie Bars recipe! Packed with M&Ms, chocolate chips, and chunks and a surprise ingredient of Reese's Peanut Butter Cups, these bars are a heavenly blend of sweet and salty, perfect for satisfying your sweet tooth.

Prep Time15

Cook Time30

Cooling time1

Total Time1 45

Course: Dessert

Cuisine: American

Diet: Vegetarian

Servings: servings

Calories: 432kcal

Author: Briana

Equipment

digital food scale
8x8 baking pan
parchment paper
hand mixer (or a stand mixer with the paddle attachment)
metal sieve
Rubber Spatula
mixing bowl

Ingredients

8 tablespoons unsalted butter, melted and slightly cooled (If not browning the butter, use 6 tablespoons melted butter, because Browning reduces the volume due to evaporation.)
¾ cup creamy peanut butter (like JIF)
¾ cup light brown sugar
½ cup granulated white sugar
1 tablespoon vanilla extract
1 large egg, room temperature
1 large egg yolk, room temperature
1 cup all purpose flour
1 cup old-fashioned oats
1 teaspoon kosher salt
½ teaspoon baking soda
¾ - 1 cup semi-sweet chocolate chips (we used mini and regular sized, use as much as you'd like!)
¾ - 1 cup dark chocolate chunks (use as many as you like, can also use moe chips!)
¾ - 1 cup M&MS (we used mini and regular sized, use as much as you'd like!)
½ cup Reese's Peanut Butter Cups, chopped (optional, about 12 mini Reese's cups)
Flaky sea salt to top (optional but very tasty)

Instructions

Preheat the oven to 350°F and grease and line an 8×8 square baking dish with parchment paper. Allow the paper to hang over the sides so you can remove the bars easily.
Optional step - skip if not browning the butter. In a small saucepan, melt 8 tablespoons of unsalted butter over medium heat. Stir constantly. The butter will begin to bubble up and get foamy, and then it will stop making so much noise. Keep a very close eye on it now, as it will brown very quickly. When the butter is a golden amber color and gives off a nutty aroma, remove it from the heat and weigh it using a food scale. It should weigh about 85 grams (or the equivalent of 6 tablespoons). This is because some of it will evaporate when cooking. Place in the fridge to cool for about 10-15 minutes before moving on. It can be warm, just not hot. See notes for our full blog post guide on how to brown butter!
In a large bowl, mix together ¾ cup of peanut butter, ¾ cup of light brown sugar, ½ cup of white sugar, 1 tablespoon of vanilla, and 6 tablespoons of melted and slightly cooled browned butter. Mix until well combined and smooth, then whisk in 1 egg and 1 egg yolk until just combined.
Sift in 1 cup of flour, 1 teaspoon of salt, and ½ teaspoon of baking soda using a metal sieve. Add in 1 cup of oats, and switch to a rubber spatula to combine the mixture gently until just barely combined. There will be some streaks of flour still visible.
Add in most of the chocolate chips/chunks, M&Ms, and optional Reese's Peanut Butter Cups. Reserve some candy for topping the bars. Stir gently to combine. with a rubber spatula, then press the cookie dough into the baking pan and top with the remaining chocolate chips, chunks, M&Ms, and Reeses's Peanut Butter Cups.
Bake for 30-35 minutes, or until the edges are golden brown and the center is evenly puffed. Sprinkle with sea salt right away, and allow the bars to cool for at least 45 minutes to 1 hour in the pan.
After the bars have cooled and set up, cut them in 12-16 pieces and enjoy!
Store the bars in an airtight container at room temperature for up to 5 days, or refrigerate for up to 10 days. To freeze, wrap tightly in plastic wrap and place in an airtight freezer-safe container for up to 3 months. Thaw overnight in the fridge.If chilled, allow them to sit out at room temperature for a few minutes before serving as they may become tough from being cold.

Notes

Check out our full guide with photos on how to brown butter!
Use room temperature ingredients. Make sure the egg and egg yolk are at room temperature for easier mixing and a smoother batter.
Measure accurately. Use a kitchen scale if you have one, as the sizes of cups vary. Accurate measurements can take a baked good from good to great!
Don't overmix. Once you add the dry ingredients to the wet, mix just until combined to avoid tough, dense bars.
Fold in mix-ins carefully. Make sure to fold in the chocolate chips, chocolate chunks, M&M's, and Reese's evenly, so every bar has a good mix of each.
Use an oven thermometer. Make sure your oven is fully preheated before baking. An oven thermometer can help confirm that the oven has reached the right temperature. Often, the beep you hear from the oven goes off prematurely before the temperature is reached. Also, different ovens run at different temperatures.
Parchment paper. Always line your baking pan with parchment paper for easier removal and cleanup. It’s also best to let the parchment hang off the sides a little bit for easy removal.
Even spreading. Use a spatula or your hands to press the dough evenly into the pan, to help ensure that the thickness is as uniform as possible. Do not pack it too tight tough.
Don’t overbake. The bars may look a bit underdone in the middle when they're ready, but they'll firm up as they cool. Don’t overbake them, or the bars will become dry!
Let the bars cool completely. Waiting for the bars to firm up is the hardest part, but it’s crucial to let the bars cool completely (for at least 45 minutes) or else they may break when attempting to get them out of the pan.
Use a heated knife for cutting bars. For clean and pretty bars, we recommend using a heated knife for slicing. You can place the knife in hot water for a few seconds, wipe it dry, then slice.
Don’t skip browning the butter. While this step is optional, it creates such a delicious depth of flavor, and we highly recommend browning it! One note, water will evaporate when browning the butter, and it should leave you with about 6 tablespoons of butter in the end, so we start with 8 tablespoons. If you want to skip this step, just use 6 tablespoons of melted butter.
Don’t forget the flaky salt! These bars are quite sweet, so the extra bit of salt balances out some of the sweetness and enhances the flavor.

Nutrition