These soft and chewy monster cookie bars are loaded with peanut butter, oats, chocolate chips, and M&M’s. It’s an easy, no-chill cookie dough that can go straight from the mixing bowl to the oven!

A stack of four monster cookie bars, surrounded by M&M's.

This easy recipe transforms my classic monster cookies into bars. The cookie dough is flavored with peanut butter and each bite is studded with oats, chocolate chips, and M&M’s.

These cookie bars are super soft, and since the dough is just pressed into a pan there’s no chilling required. This recipe also uses just one mixing bowl, so they’re incredibly easy to make with minimal cleanup.

These monster cookie bars would be perfect for any holiday parties or cookie exchanges you might have coming up. You can also use seasonal M&M’s in this recipe to make them suit any occasion!

An overhead view of the ingredients used in monster cookie bars.

Ingredient Notes

  • Peanut Butter: You’ll want to use a creamy, no-stir peanut butter in this recipe. Something like Jif or Skippy would be perfect.
  • Oats: I’ve tested this recipe with old-fashioned rolled oats and quick oats, and both worked just fine! I personally prefer to use rolled oats in this recipe because it makes the bars a little chewier.
  • Light Brown Sugar: Keeps the bars moist and chewy, while also adding more flavor. If you’re out, you can make your own brown sugar using some granulated sugar and molasses.
  • Eggs: This recipe calls for one whole egg plus one egg yolk. I love to add an extra egg yolk to cookie recipes whenever possible because it makes them richer and softer. You can save the extra egg white and use it in candied almonds, candied pecans, or your morning egg scramble.
  • M&M’s: I used regular M&M’s but you can use any color you prefer.
  • Chocolate Chips: I used semi-sweet chocolate chips in this recipe to prevent the bars from tasting too sweet.
Melted butter and peanut butter in a glass mixing bowl with a whisk.
Melted butter, peanut butter, eggs, and brown sugar in a glass mixing bowl with a whisk.

How To Make Monster Cookie Bars

  1. Combine the wet ingredients: Whisk together the peanut butter, melted butter, and brown sugar in a large mixing bowl.
  2. To the peanut butter mixture, whisk in the egg, egg yolk, and vanilla.
Peanut butter oatmeal cookie dough in a glass mixing bowl with a rubber spatula.
Monster cookie dough in a glass mixing bowl with a rubber spatula.
  1. Add the dry ingredients: To the same mixing bowl, mix in the flour, oats, baking soda, and salt.
  2. Fold in the mix ins: Use a rubber spatula to gently fold in the M&M’s and chocolate chips.
An overhead view of unbaked cookie dough in a baking dish.
An overhead view of monster cookie bars in a baking dish.
  1. Transfer the cookie dough to a baking dish: You’ll want to use a 9×13-inch baking dish that’s been lined with parchment paper and sprayed with non-stick cooking spray. Make sure to press the cookie dough down and smooth it out into one even layer.
  2. Bake: You’ll know the bars are done when the top looks set (no longer wet) and it’s lightly browned. The bars will be soft when they first come out of the oven, so make sure to let them cool completely before removing from the pan and slicing.
An overhead view of sliced bar cookies arranged on parchment paper.

Recipe Variations

I kept these bars as classic as possible, but you can easily make adjustments to make these your own! Here are a few ideas:

  • Peanut Butter: Can be replaced with another no-stir nut or seed butter.
  • M&M’s: Use seasonal M&M’s to suit any holiday, like red and green for Christmas, red and blue for 4th of July, etc. You can even replace the M&M’s with mini peanut butter cups.
  • Chocolate Chips: I used semi-sweet chocolate chips, but milk, dark, or even white chocolate chips may be used. If you’re a huge peanut butter fan, peanut butter chips would be delicious too!
A close up view of a monster bar cookie next to a dish of M&M's.

Baking Tips

  • I suggest lining your baking dish with parchment paper and leaving some overhang. This will give you something to grip so you can remove the bars from the pan more easily.
  • Check to make sure that you’re using a no-stir, “traditional” peanut butter. Natural peanut butter will make the bars greasy and won’t work as well in this recipe.
  • Don’t over bake the bars! The tops should be set, and a toothpick inserted in the middle of the pan should come out mostly clean. The bars will be soft when they first come out of the oven, but they will continue to firm up as they cool.
A stack of three monster cookie bars.

Monster Cookie Bars

Prep Time: 20 minutes
Cook Time: 25 minutes
Total Time: 45 minutes
These soft and chewy monster cookie bars are loaded with peanut butter, oats, chocolate chips, and M&M's. It's an easy, no-chill cookie dough that can go straight from the mixing bowl to the oven!
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Ingredients

Servings: 24 bars
  • ¾ cup unsalted butter melted and cooled for 10 minutes (170 grams; 1.5 sticks)
  • ¾ cup creamy peanut butter (190 grams) avoid natural pb
  • 1 ½ cups packed light brown sugar (300 grams)
  • 1 large egg at room temperature
  • 1 large egg yolk at room temperature
  • 1 teaspoon pure vanilla extract
  • 1 ½ cups all-purpose flour spooned & leveled (190 grams)
  • 1 ½ cups old-fashioned rolled oats (150 grams)
  • ½ teaspoon baking soda
  • ¼ teaspoon salt
  • 1 cup M&M’s (215 grams)
  • ¾ cup semi-sweet chocolate chips (135 grams)

Instructions
 

  • Preheat the oven to 350°F (177°C).
  • Line a 9×13-inch pan with foil or parchment paper, leaving some overhang for easy removal. Spray with nonstick cooking spray and set aside.
  • In a large mixing bowl, whisk together the melted butter and peanut butter until well combined, then whisk in the brown sugar.
  • Mix in the egg, egg yolk, and vanilla extract until just combined.
  • Add the flour, oats, baking soda, and salt and stir until well combined, then gently fold in the M&M’s and chocolate chips.
  • Scoop the cookie dough into the prepared pan and press it down into one even layer. Top with a few extra M&M’s and chocolate chips, if desired.
  • Bake for 23 to 28 minutes or until the tops of the bars are set and very lightly browned. A toothpick inserted into the center should come out mostly clean with just a few moist crumbs.
  • Remove from the oven and transfer to a wire rack to cool completely in the pan.
  • Once the bars have cooled, lift them out of the pan using the overhang from the foil/parchment paper. Slice into bars and enjoy!

Notes

Storage Instructions: Bars may be stored in an airtight container at room temperature for up to one week.
Freezing Instructions: Bars will freeze well for 2 to 3 months. Thaw to room temperature before serving.
Cuisine: American
Course: Dessert
Author: Danielle Rye
Did you make this recipe?Mention @livewellbakeoften on Instagram or tag #livewellbakeoften.

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