Gluten-Free Monster Cookie Bars

Soft and chewy gluten-free monster cookie bars are packed with oats, peanut butter, chocolate chips, and M&Ms. Baking them all at once on a sheet pan creates a whole batch of cookies with a fraction of the effort.

Gluten-Free Sheet Pan Monster Cookies stacked on a plate.
Kalisa Marie Martin

Ever heard of monster cookies? To know them, is to love them.

If you are familiar, you likely have connections to the Midwest, where they’re a favorite. Imagine oatmeal peanut butter cookies studded with chocolate chips and M&Ms. Once you try them, you’ll agree that they should be making them in all 50 states!

In general, cookies are great for feeding a crowd because one recipe yields dozens, but scooping, shaping, and baking them in batches takes time and effort! If you spread your entire bowl of cookie dough out on a sheet pan instead, you can get the job done in one go. Then, all you have to do is cut them into squares after they’ve baked.

Not only is this recipe a breeze for the baker, but it is a dream for whoever gets to eat them. These cookies are soft, chocolatey, and nutty! The oats add a great chewiness, the chocolate chips add to the melty gooey texture, and the M&Ms maintain their shape and crunch.

Also, did I mention that these are gluten-free? With all of these flavors and textures, you won’t be able to tell the difference. 

The Best Pan for Making Bar Cookies

For sheet pan cookies, we need to use a pan with higher edges that will contain the cookie dough as it rises in the oven and a jelly roll pan is perfect for this. A jelly roll pan is smaller than a half-sheet pan; 15x10 inches in size and has 1-inch higher sides so there won’t be any overflow of dough as it bakes in the oven.

Tip! Lining the pan with parchment paper leaving an overhang on the two long sides is key. This will allow you to easily remove the entire sheet pan cookie to a board before cutting into squares. Just make sure you let the cookie cool completely before lifting.

Close up of an overhead view of Bar Cookies of the Gluten Free Monster Variety.
Kalisa Marie Martin

Why Monster Cookies are the Perfect Gluten Free Snack?

Whenever a cookie recipe has multiple elements—nuts, chips, and more contributing to its texture versus relying predominantly on flour it’s a great candidate for converting to gluten-free.

In this case, the oats add body and chew, peanut butter adds creaminess, and the chocolate chips and M&Ms add gooeyness and crunch, respectively.

This recipe only calls for one cup of gluten-free flour and with everything else going on, no one will miss the gluten. I use King Arthur Measure for Measure Flour, but you can use whichever is your go-to gluten-free blend.

Monster Cookie Swaps and Substitutions

There’s more than one way to make a monster cookie and you can take this recipe and make it your own. Just be sure that the mix-ins stay around 2 cups total.

  • Try half M&Ms and half Reese’s Pieces.
  • Throw in some chopped peanuts!
  • If you have a peanut allergy swap peanut butter for almond butter or even sunflower seed butter to avoid nuts all together. (But stick to creamy versus crunchy and avoid any brands that are 100% natural. Natural nut butters will actually lead to a drier cookie.)
  • Try crushed pretzels or potato chips.
The BEST GF Monster Cookie Recipe sliced into squares and set on parchment paper.
Kalisa Marie Martin

 A Note About Oats

This recipe calls for quick cooking oats as the finer texture helps to bind the cookie dough together. If all you have is old fashioned oats, pulse them a few times in a food processor to break them down. Your cookies will have a crumbly texture if you use old fashioned oats without pulsing them first.

How to Store the Cookies

Store these gluten-free monster cookies in an airtight container at room temperature. They will stay chewy for about 5 days. You can also freeze them between layers of parchment and sealed in a ziptop bag for up to 3 months to snack on as needed.

Gluten-Free Sheet Pan Monster Cookies stacked on a plate.
Kalisa Marie Martin.

More Amazing Gluten-Free Treats

Gluten-Free Monster Cookie Bars

Prep Time 10 mins
Cook Time 15 mins
Total Time 25 mins
Servings 24 cookie squares

Use your favorite all-purpose gluten-free flour blend and be sure to use gluten-free oats. To make this recipe you will need a 15 x 10 x 1 inch jelly roll pan (rimmed baking sheet).

Ingredients

  • 1 cup all-purpose gluten-free flour
  • 1 cup gluten-free quick oats
  • 1/2 teaspoon salt
  • 1/2 teaspoon baking soda
  • 1/2 cup unsalted butter, melted and cooled
  • 1 cup light brown sugar, packed1/2 cup creamy peanut butter
  • 1 large egg
  • 1 teaspoon vanilla extract
  • 1 cup M&Ms
  • 1 cup chocolate chips

Method

  1. Preheat the oven and prepare the baking pan:

    Preheat the oven to 350°F. Line the jelly roll pan with parchment paper leaving a 1-inch overhang on the two long sides. You will use this later to lift the bars out of the pan.

    Lining a pan with parchment to make Bar Cookies of the Gluten Free Monster Variety
    Kalisa Marie Martin
  2. Combine the dry ingredients:

    In a medium bowl, whisk together gluten-free flour, gluten-free oats, salt, and baking soda.

    Adding flour to a bowl to make Bar Cookies of the Gluten Free Monster Variety.
    Kalisa Marie Martin
    Whisking dry ingredients in a bowl to make Gluten-Free Sheet Pan Monster Cookies.
    Kalisa Marie Martin
  3. Combine the wet ingredients:

    In a separate large bowl, whisk together the butter, brown sugar, peanut butter, egg, and vanilla until completely smooth.

    Whisking sugars and egg in a bowl to make Gluten-Free Sheet Pan Monster Cookies.
    Kalisa Marie Martin
    Whisking sugars and egg in a bowl to make Gluten-Free Sheet Pan Monster Cookies.
    Kalisa Marie Martin
  4. Combine the wet and dry ingredients:

    Add the dry ingredients to the bowl of wet ingredients and stir to combine. Reserve 2 tablespoons each of the M&Ms and chocolate chips. Add the remainder to the cookie dough and stir to combine

    Adding flour mixture to wet mixture to make Gluten-Free Sheet Pan Monster Cookies.
    Kalisa Marie Martin
    Adding flour mixture to wet mixture to make Gluten-Free Sheet Pan Monster Cookies.
    Kalisa Marie Martin
    Adding flour mixture to wet mixture to make Gluten-Free Sheet Pan Monster Cookies.
    Kalisa Marie Martin
    Dough for The BEST GF Monster Cookie Recipe in a bowl with a spoon.
    Kalisa Marie Martin
  5. Transfer dough to prepared baking pan:

    Turn the dough out onto the prepared baking pan. Place a sheet of parchment paper on top and use your hands to gently press it evenly into the pan. Remove the top piece of parchment. Sprinkle the reserved M&Ms and chocolate chips on top, pressing them just slightly into the dough

    Dough for The BEST GF Monster Cookie Recipe in a baking pan.
    Kalisa Marie Martin
    Pressing dough with parchment paper with hands to make Gluten-Free Sheet Pan Monster Cookies.
    Kalisa Marie Martin
    Gluten-Free Sheet Pan Monster Cookies ready to be baked.
    Kalisa Marie Martin
  6. Bake cookie bars:

    Bake the cookie bars until the dough has risen and the edges have just begun to turn slightly golden, 15 to 17 minutes. The middle may appear under done, but it will set as it cools.

    Gluten-Free Sheet Pan Monster Cookies baked in pan.
    Kalisa Marie Martin
  7. Cool the cookie bars:

    Cool in the baking pan on a cooling rack for at least 1 hour.

  8. Cut and serve the cookie bars:

    Using the sides of the parchment as a sling, transfer the sheet pan cookie to a board and cut into 24 bars and serve.

    Removing Gluten-Free Sheet Pan Monster Cookies from the pan by holding the parchment.
    Kalisa Marie Martin
    Sliced Gluten-Free Sheet Pan Monster Cookies.
    Kalisa Marie Martin