Monster Cookie Bars

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Monster Cookie Bars are the chewiest, most flavor-packed cookie bars ever. Perfect for all the peanut butter lovers out there!

Lately, I’ve been all about cookie bars. There’s something really satisfying about biting into a thick, chewy cookie bar – especially when it’s loaded with peanut butter and chocolate!

If I’m not bringing my go-to Best Ever Chewy Brownies to a party or potluck, it’s because I’m bringing cookie bars. Recipes like my Peanut Butter Toffee Chocolate Chunk Bars or Chocolate Frosted Sugar Cookie Bars are so easy to transport and serve and don’t take up precious fridge space. 

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These Monster Cookie Bars are going to be a staple in my holiday baking! They’re naturally gluten-free (when you buy certified gluten-free ingredients), so they’re an easy solution if you’re serving a crowd with dietary restrictions. 

Because, really, who can resist a peanut butter-packed cookie bar loaded with oats, chocolate chips, and M&Ms? They’re a guaranteed crowd-pleaser!

Sprinkle of Science

How to Make Monster Cookie Bars

No Flour in This Monster Cookie Bars Recipe? 

Yes, these Monster Cookie Bars are flourless cookie bars. There’s enough structure provided by the peanut butter, oats, and egg + egg yolk to hold everything together and bake into a chewy, delicious cookie bar. 

Gluten free note: Make sure to use certified gluten-free oats if serving to someone with a severe gluten allergy. Some people with celiac cannot tolerate even gluten-free oats.

What Type of Peanut Butter for Monster Cookie Bars? 

I recommend using natural peanut butter here – just be sure to stir it very well before using to avoid any dry/oily pockets. My favorite is Kirkland Organic (it’s cheaper at Costco!). Natural peanut butter provides a much bolder PB flavor in these Monster Cookie Bars. If you don’t have any, conventional peanut butter (Jif / Skippy) will work too. 

What Type of Oats for Monster Cookie Bars?

Use old-fashioned rolled oats, not quick oats, which can make the cookie bars less chewy and more mushy. Be sure to measure correctly, as accidental over-measuring can cause dry, hard cookie bars. Learn more about how to measure ingredients correctly here. If you intend to make this flourless recipe fully gluten free, use certified gluten-free oats.

Do I Have to Use Dark Brown Sugar?

For the best flavor and moisture, as well as additional chewiness, I recommend sticking with dark brown sugar. Feel free to substitute light brown sugar at a 1:1 ratio, but note that your cookie bars may turn out lighter in color and slightly less moist and chewy.

Can I Use Almond Butter Instead? 

I haven’t tested that specifically in this recipe, but a 1:1 swap of almond butter for peanut butter works perfectly in my Peanut Butter Chocolate Chip Cookies recipe. I’d love to hear how it turns out if you try it here!

Can I Change up the Mix-Ins?

What’s the Best Pan for Monster Cookie Bars?

My favorite pan for brownies, blondies, and cookie bars is this light-colored metal 8 by 8-inch pan. I do not recommend using glass or ceramic pans for cookie bars. Learn more about Glass vs. Metal Baking Pans here.

Can I Double This Recipe? 

Yes! Use a 9 by 13-inch metal pan like this one and double all ingredients. Bake at the same temperature, adding a couple minutes to the bake time.

Can I Make Monster Cookies Instead?

I have a recipe for that! Check out my Monster Cookies recipe here.

How to Store Monster Cookie Bars?

Monster Cookie Bars can be stored in an airtight container at room temperature for up to 3 days. Cookie bars will soften more the longer they sit. Store with a tortilla to keep the cookie bars from drying out. 

Can You Freeze Monster Cookie Bars?

Freeze the baked, cooled cookie bars as a whole slab or individual pieces wrapped well in plastic wrap and stored inside an airtight container. Freeze for up to one month. To thaw, allow to sit at room temperature for a few hours. 

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How to Make

Monster Cookie Bars

By: Tessa Arias

5 from 1 vote

Yield: 16 cookie bars

Prep Time: 20 minutes mins

Cook Time: 23 minutes mins

Total Time: 43 minutes mins

Review Print SaveSaved!

Monster Cookie Bars are gooey, chewy, and ultra-thick, packed with peanut butter flavor, and loaded with chocolate chips and M&Ms. These cookie bars are easy to make, ready in under 45 minutes!

Ingredients 

• ▢ 4 tablespoons (57 grams) unsalted butter, at cool room temperature
• ▢ 3/4 cup (203 grams) natural peanut butter, very well stirred*
• ▢ 1/2 cup (100 grams) granulated sugar
• ▢ 1/2 cup (100 grams) lightly packed dark brown sugar**
• ▢ 1 large egg plus 1 egg yolk, at cool room temperature
• ▢ 1½ teaspoons vanilla extract
• ▢ 2 cups (198 grams) old-fashioned rolled oats
• ▢ 1 teaspoon baking soda
• ▢ 1/4 teaspoon salt
• ▢ 1/2 cup (85 grams) semisweet chocolate chips, plus extras to top cookie bars, optional
• ▢ 1 cup (200 grams) M&M candies, plus extras to top cookie bars, optional
• ▢ Flaky sea salt, for topping, optional

Instructions

• Preheat the oven to 350°F. Line an 8 by 8-inch metal baking pan with foil or parchment paper. Spray with nonstick cooking spray.
• In the bowl of an electric mixer, beat the butter, peanut butter, granulated sugar, and brown sugar on medium-high speed until smooth and creamy, 2 to 3 minutes. Scrape down the bottom and sides of bowl. Beat in the egg, egg yolk, and vanilla. Add the oats, baking soda, and salt, mixing on low speed until combined. Fold in the chocolate chips and M&Ms with a rubber spatula.
• Press dough evenly into prepared pan. Optional: Dot the tops of the bars with additional M&Ms and/or chocolate chips before baking.
• If time permits, cover pan with plastic wrap and refrigerate for 24-48 hours to yield a chewier, more flavorful bar.
• Bake for about 23-25 minutes, or until the bars no longer look wet on top, the center appears set, and the top is golden brown. Set pan on a wire rack and sprinkle flaky sea salt on top, if desired. For best texture and to allow the peanut butter flavor to fully develop, let bars sit until completely cooled before slicing and enjoying. It’s worth it, promise!
• Cookie bars can be stored in an airtight container at room temperature for up to 3 days.

Show us!If you make this recipe, be sure to snap a picture and share it on Instagram with #handletheheat so we can all see!

Recipe Notes

*We recommend using natural peanut butter that’s been very well stirred as it will yield a much bolder peanut butter flavor; however, conventional peanut butter (Skippy and Jif) work well too. You can alternatively use almond butter at a 1:1 ratio.

**Using dark brown sugar will yield a chewier, richer cookie bar but may be substituted at a 1:1 ratio with light brown sugar.

Please note that this recipe does not require any flour, making these cookie bars a great gluten-free option (if using certified gluten-free ingredients).

This post was originally published in 2013 and has been updated with recipe improvements, additional baking tips, and new photos. Photos by Ashley Barbey.