Chewy Monster Cookie Bars Recipe with M&M’s and Chocolate Chips

Monster cookie bars are incredibly simple to make. This sweet, chewy treat is an ideal way to use up leftover candy and comes together quickly to please little monsters and big kids alike.

Halloween is just around the corner, and many stores have already stocked shelves with seasonal candy. If your stash of minis and novelty chocolates starts to multiply, monster cookie bars are a fantastic solution.

These bars accept a wide variety of mix-ins, from mini M&M’s to chopped candy bars, so you can transform leftover Halloween treats into a big, shareable pan of cookie goodness.

WHAT ARE MONSTER COOKIE BARS?

Classic Monster cookies combine peanut butter, oats, chocolate chips, and M&M’s, creating a hybrid of peanut butter, oatmeal, and candy-studded cookies. Monster cookie bars take those flavors and textures and pack them into an easier, sliceable pan bar.

The result is thick, chewy, slightly gooey bars that highlight peanut butter and oats while showcasing whichever candies you choose to toss in. Chopped Snickers, Reese’s Cups, Twix pieces, or any seasonal minis would be excellent additions.

HOW TO MAKE MONSTER COOKIE BARS

Cookie bars are generally easier and quicker than shaping individual cookies. This recipe is forgiving and customizable—use whatever mix-ins you have on hand for a delicious pan every time.

Add-in options

Along with a cup of chocolate chips, these bars call for about a heaping cup of mix-ins. Traditional M&M’s work well, but you can mix and match to use up candy or pantry items.

Try any of these:

• Classic M&M’s
• Reese’s Pieces
• Mini Reese’s Cups
• Salted peanuts
• Mini pretzels
• Raisins
• Chopped mini candy bars (Snickers, Twix, etc.)
• Peanut M&M’s or Caramel M&M’s

Use one cup of a single add-in or combine several to reach a heaping cup. This is an easy, tasty way to put leftover Halloween candy to good use.

What kind of peanut butter and oats should you use?

Use smooth, creamy peanut butter for the best texture. Regular creamy peanut butter or a no-stir natural variety works fine, but avoid coarse, freshly ground peanut butter from the store—it can be grainy and less suitable for baking.

Always use old-fashioned rolled oats in this recipe; quick-cooking oats will change the texture and are not recommended.

Making the bars

Preheat the oven to 350°F (175°C). Line a 9×13-inch pan with foil, leaving about an inch of foil hanging over the sides to create handles for easy removal. Lightly spray the foil with nonstick cooking spray.

Cream ½ cup unsalted butter (room temperature), 1½ cups creamy peanut butter, 1 cup packed light brown sugar, and 1 cup granulated sugar together on medium speed for about 3 minutes until light and fluffy. Add 2 teaspoons pure vanilla extract and 3 large eggs and mix to combine.

In a separate bowl, whisk together 1 cup all-purpose flour, 2 teaspoons baking soda, 1 teaspoon kosher salt, and 3¼ cups old-fashioned oats. Gradually add the dry ingredients to the butter mixture, then stir in 1 cup semisweet or milk chocolate chips and about 1 heaping cup of your chosen add-ins.

Spread the batter into the prepared pan in an even layer. If desired, press a few extra candies or chips lightly onto the top for visual appeal. Bake for 30–35 minutes. For the best texture, aim to slightly under-bake rather than over-bake so the bars remain chewy and moist.

Optionally, sprinkle flaky sea salt over the warm bars as soon as they come out of the oven. Let the pan cool completely before lifting the foil handles to remove the bars and cutting them into squares.

STORAGE TIPS

Store the bars in an airtight container at room temperature for up to 3 days. For longer storage, freeze them in an airtight container with parchment or wax paper between layers.

Thaw at room temperature, or reheat individual bars in the microwave for about 30 seconds to enjoy a warm, gooey treat.