Preheat the oven to 350°F and line a 9 x 9 pan with parchment paper on all sides. Set aside.
Prepare the brownies according to the directions on the box. Pour the batter into the prepared pan, using a rubber spatula to spread the brownie batter into an even layer.
Place the peanut butter in a microwave safe bowl and melt for 30 seconds. Set aside. Place the fluff in a microwave safe bowl and microwave for 20 seconds.
Use a large spoon to drop dollops of fluff and peanut butter in a checkerboard fashion. You don’t need to be precise about it. Using a butter knife, make big swirls of peanut butter and fluff. Do not over swirl the brownies–you’ll want undisturbed ribbons of marshmallow fluff and peanut butter.
Place in the oven to bake for 20 to 30 minutes or until the edges are set and a knife inserted into the middle comes out mostly clean.
Allow the brownies to cool completely before slicing into 16 small brownies or 9 huge brownies. Enjoy!
Notes
You can also feel free to make homemade brownies for this recipe!