Monster Cookie Bars
These monster cookie bars are soft, chewy, and fully loaded with peanut butter, oats, semisweet chocolate chips, and colorful M&M candies. Every bite is rich, sweet, and packed with texture—just the kind of treat that brings a little nostalgia with it. Easy to make and perfect for sharing, they’re a crowd-pleasing dessert that never goes out of style.
Monster Cookie Bars
These monster cookie bars are serving serious Goonies energy—messy, adventurous, nostalgic, fully loaded, sweet, and totally unforgettable.
The bars are soft, chewy, and packed with peanut butter, oats, chocolate chips, and M&M candies, and basically the treasure you’ve been searching for. One bite and you’ll be exclaiming, “Goonies never say die,” because there’s no way you’re stopping at just one.
Whether you’re baking for a party or just channeling your inner Chunk (“Hey you guys!”), these bars bring all the fun. Honestly, they’re so good, you might start guarding the pan like it’s One-Eyed Willy’s treasure.
If these cookie bars don’t scare you, check out my brookie bars, chocolate chip cookie bars, and sugar cookie bars.
Why You Will Love This Recipe
- Easy to make: Simple ingredients and no-fuss steps make this a great go-to recipe.
- Soft & chewy texture: Every bite is perfectly tender with just the right amount of chew.
- Fully loaded: Packed with peanut butter, oats, chocolate chips, and M&M candies for maximum flavor and texture.
- Sweet & nostalgic: A fun, throwback-style treat that feels comforting and familiar.
- Perfect for sharing: Great for parties, bake sales, or anytime you need a crowd-pleasing dessert.
Recipe Ingredients
- All-purpose flour
- Cornstarch – A bit of cornstarch helps to keep the bars soft and chewy.
- Baking soda + salt
- Unsalted butter – Room temperature.
- Light brown sugar + granulated sugar – This combination of sugars adds the perfect amount of sweetness and gives the bars their classic chewy texture.
- Eggs + egg yolk – Room temperature.
- Vanilla extract
- Creamy peanut butter – Use regular peanut butter for the best results.
- Quick oats – You can substitute with old-fashioned rolled oats if preferred.
- Semisweet chocolate chips – You could also use milk, dark, white, or peanut butter chips.
- M&M candies – Use regular, mini, or combination of both.
How to Make Monster Cookie Bars
Make the peanut butter cookie dough.
Fold in the oats, semisweet chocolate chips, and M&M candies.
Spread the monster cookie dough evenly into a prepared 9×13 inch baking pan.
Press additional chocolate chips and M&M candies into the top of the cookie dough.
Bake the monster cookie bars.
Cool the cookie bars completely and then cut into squares.
Courtney’s Expert Bar Cookie Baking Tips
- Line the baking pan with parchment paper extending past the sides of the pan for easy removal.
- Use regular creamy peanut butter. Stick with a standard creamy peanut butter (like Jif or Skippy) for the best texture; natural peanut butter can make the bars dry or crumbly.
- Press extra chocolate chips and M&Ms into the top of the dough before baking for a bakery-style finish. Use different colors of M&Ms to make the bars more festive for Christmas, Halloween, Valentine’s Day or any holiday.
- Bake the bars at a lower temperature for a longer period of time for a more even rise.
- Don’t overbake the bars. The bars should look slightly underdone in the center when you take them out of the oven. They will continue to set as they cool and stay soft and chewy.
- Remove the bars from the pan to make cutting easier. Chill the bars for 1 hour before cutting to make it even easier. Clean your knife in between slices for perfect cuts.
Frequently Asked Questions
I don’t recommend using natural peanut butter as it can make the bars dry or crumbly. A regular creamy peanut butter (Jif, Skippy, or generic brand) works best for structure and texture.
Yes! Use old-fashioned rolled oats if you prefer more texture in your cookie bars. Either will work just fine in this recipe.
Definitely! Use different varieties or chocolate chips or M&Ms to customize these bars to your liking. Just stick with the amounts in the recipe card for the perfect ratio of cookie to mix-ins.
For the cookie bars pictured, I cut them into 12 pieces for photos—these are quite large! For serving, I suggest cutting them into 24 pieces instead.
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Monster Cookie Bars
Ingredients
- 2 cups (250 g) all-purpose flour
- 2 tsp (2 tsp) cornstarch
- 1 tsp (1 tsp) baking soda
- 1 tsp (1 tsp) salt
- 1 cup (227 g) unsalted butter (melted)
- 1 ½ cups (330 g) light brown sugar
- ½ cup (100 g) granulated sugar
- 2 (2) large eggs (room temperature)
- 1 (1) egg yolk (room temperature)
- 2 tsp (2 tsp) vanilla extract
- 1 cup (260 g) creamy peanut butter
- 1 cup (80 g) quick oats (substitute with old-fashioned rolled oats)
- 1 ¼ cups (225 g) semisweet chocolate chips (divided – 1 cup for the cookie dough & ¼ cup for the top of the bars)
- 1 ½ cups (310 g) M&M candies (divided – 1 ¼ cups for the cookie dough & ¼ cup for the top of the bars)
Instructions
- Preheat the oven to 340° F (171℃). (Note: We are baking at a slightly lower temperature to better control the rise of the bars). Prepare a 9×13 baking pan with nonstick cooking spray and parchment paper. Cut the parchment paper so that it extends up past the sides of the pan.
- In a medium mixing bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt. Set aside.
- In the bowl of a stand mixer fitted with a paddle attachment or in a large bowl with an electric hand mixer, mix together the melted unsalted butter, light brown sugar, and granulated sugar until well combined and smooth. Add in the eggs and vanilla extract and mix until well combined. Then, add in the peanut butter and mix until fully incorporated and smooth.
- Add the dry ingredients and mix on low speed until the cookie dough forms. Fold in the quick oats, 1 cup of semisweet chocolate chips and 1 ¼ cups of M&M candies until they are well dispersed throughout the cookie dough.
- Use an offset spatula to spread the cookie dough into an even layer in the prepared 9×13 baking pan. Press the additional ½ cup of chocolate chips and ½ cup of M&Ms into the top of the cookie dough.
- Bake the monster cookie bars for about 35-37 minutes until the edges of the bars are golden-brown and the center is very slightly underbaked. The bars will finish baking as they cool in the pan. Cool the bars completely to room temperature before removing them from the pan and cutting into squares.
Notes
- Room temperature: Store the monster cookie bars in an airtight container at room temperature for 2 to 3 days. Serve at room temperature or heat the bars in the microwave for 10 seconds to melt the chocolate chips and M&Ms.
- Refrigerator: I don’t recommend storing the monster cookie bars in the refrigerator as they will get hard and dry out.
- Freezer: Store the monster cookie bars individually wrapped in an airtight container for up to 3 months. Thaw to room temperature before serving.
Nutrition
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About Courtney
Baker | 80s Lover | Cookbook Author
Courtney is a self-taught home baker and cookbook author of “Girls Just Wanna Bake Cupcakes”. She loves to create new and unique recipes while simultaneously listening to 80s music.
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