Monster Cookie Bars

Preparation - 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.

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

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

Combining - 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.

Assembly - 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.

Baking - 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.

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

Serving and Storage - Using the sides of the parchment as a sling, transfer the sheet pan cookie to a board and cut into 24 bars and serve. Store 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 zip-top bag for up to 3 months.