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With or without ice cream and hot fudge, these Monster Cookie Bars are sure to be a hit! Peanut butter, M&Ms, oatmeal…they’re the trifecta of perfect cookie bar. These are better than pioneer woman and taste of home combined!

stack of monster cookie bars


Have you ever had a monster cookie? It’s a peanut butter oatmeal cookie that’s filled with chocolate chips and M&Ms. I love them, because they’re a combination of two of my favorite cookie recipes. But as much as I like cookies, I think I like cookie bars better because there is no scooping. I just press the dough in the pan and bake it, easy peasy.

This recipe starts with my favorite peanut butter cookie recipe. I adapted it to add oats to make it MONSTER. And, of course, I added chocolate chips, regular M&Ms, and PEANUT BUTTER M&M’s. I swear, peanut butter M&M’s are my downfall. I can easily eat a 1 lb. bag all by myself!

Once you bake them they become a soft and gooey cookie bar that’s perfect for when you’re craving peanut butter and oatmeal cookies together.

Ingredients in MONSTER COOKIE BARS

Ingredients Needed

  • Butter: As always, I start with unsalted butter that’s been softened. Learn to soften butter quickly!
  • Peanut Butter: A must for monster cookies! You can use creamy or crunchy, but use a no-stir brand.
  • Sugar: Both granulated and brown sugar makes a soft and chewy bar.
  • Flour: I always use all-purpose flour, be sure to measure it correctly.
  • Quick Cooking Oats: I prefer baking with quick oats. To make these bars with old fashioned oats you just need to pulse the oats in a food processor to break them down – learn to make your own quick oats!
  • Mix-Ins: Chocolate Chips and M&Ms

Be sure to see the recipe card below for full ingredients & instructions!

How to make Monster Cookie Bars

  1. Beat butter and peanut butter with a hand or a stand mixer until smooth. Mix in both sugars until creamy.
  2. Add eggs, vanilla, baking soda, and salt and mix until smooth.
  3. Slowly mix in the flour and oats. Stir in the chocolate chips and M&Ms.
  4. Press cookie dough into the prepared pan. If desired, sprinkle more M&Ms on top for decoration. Bake for 22-25 minutes in an oven preheated to 350°F.
stack of cookie bars on white plate with M&Ms and Ala mode

Variations and Expert Tips

  • Make Christmas Monster Cookie Bars with holiday M&Ms
  • Reese’s Pieces Monster Cookie Bars would be fantastic – just swap the M&Ms for Reese’s Pieces!
  • Do not over bake these – they’re best a little under baked and gooey
  • How to serve Monster Bars: I love serving these plain, just as a bar cookie, or topping with ice cream!
stack of cookie bars

Monster Cookie Bars FAQ

How long to bake monster cookie bars?

Bake these 22-25 minutes or until they just lose their glossy sheen.

How to store cookie bars? Can you freeze monster cookie bars?

Store these in an airtight container. Freeze for up to 3 months.

Can you make monster cookie bars into cookies?

It’s not an exact swap so I suggest making gluten free monster cookies instead.

stack of monster cookie bars

Monster Cookie Bars

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Monster Cookie Bars – this easy bar cookie recipe is full of peanut butter, oats, and tons of M&Ms! EVERYONE loves these, especially with ice cream!
Prep Time 15 minutes
Cook Time 30 minutes
Total Time 45 minutes
Yield 24 bars
Serving Size 1 bar

Ingredients
 

  • ½ cup (113 g) unsalted butter softened
  • 1 cup (268 g) peanut butter
  • ¾ cup (150g) packed brown sugar
  • ½ cup (100g) granulated sugar
  • 2 large eggs
  • 1 teaspoon vanilla extract
  • ½ teaspoon baking soda
  • ½ teaspoon salt
  • ¾ cup (93g) all-purpose flour
  • 3 cups (240g) quick cooking oats
  • 1 cup (170g) chocolate chips
  • 2 cups M&Ms any kind, I used regular and peanut butter
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Instructions

  • Preheat oven to 350°F. Line a 9×13” pan with foil and spray with nonstick cooking spray.
  • Beat butter and peanut butter with a hand or a stand mixer until smooth. Mix in both sugars until creamy. Add eggs, vanilla, baking soda, and salt and mix until smooth, then slowly mix in the flour and oats. Stir in the chocolate chips and M&Ms.
  • Press cookie dough into the prepared pan. If desired, sprinkle more M&Ms on top for decoration. Bake for 22-25 minutes or just until the edges are golden brown. Cool before cutting into bars.
  • These are great served as bars or as an ice cream sundae! Store in an airtight container for up to 3 days or freeze for up to one month.

Recipe Video

Recipe Notes

  • Make Christmas Monster Cookie Bars with holiday M&Ms
  • Reese’s Pieces Monster Cookie Bars would be fantastic – just swap the M&Ms for Reese’s Pieces!
  • Do not over bake these – they’re best a little under baked and gooey
  • How to serve Monster Bars: I love serving these plain, just as a bar cookie, or topping with ice cream!

Recipe Nutrition

Serving: 1bar | Calories: 322kcal | Carbohydrates: 40g | Protein: 6g | Fat: 16g | Saturated Fat: 7g | Polyunsaturated Fat: 2g | Monounsaturated Fat: 4g | Trans Fat: 1g | Cholesterol: 28mg | Sodium: 146mg | Potassium: 126mg | Fiber: 2g | Sugar: 28g | Vitamin A: 194IU | Vitamin C: 1mg | Calcium: 48mg | Iron: 1mg
Nutritional information not guaranteed to be accurate
Course Dessert
Cuisine American

Favorite Cookie Bar Recipes



Dorothy Kern

Welcome to Crazy for Crust, where I share recipes that are sometimes crazy, often with a crust, and always served with a slice of life.

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20 Comments

  1. I attempted to make the recipe and was so excited to see weighted measurements! However, the weight for the oats does not match the cups measurement. I measured 2x and 256g=2 cups 🙁

  2. Looks yummy! I think it shows you’re a good mom that you’re thinking about these things!