Step 1: Prepare the Pan and Batter
Preheat your oven to 350°F. Line a 9×9 inch baking pan with parchment paper, allowing the paper to overhang the edges to create “slings.” Prepare the brownie batter exactly according to the directions on the box. Pour the prepared batter into the parchment-lined pan and spread it into an even layer using a rubber spatula.

Step 2: Prepare the Swirls
Place the creamy peanut butter in a microwave-safe bowl and heat for 30 seconds until melted and easily pourable. In a separate microwave-safe bowl, place the marshmallow fluff and microwave for 20 seconds to make it loose and pliable.

Step 3: Swirl the Toppings
Using a large spoon, drop dollops of the melted peanut butter and the melted marshmallow fluff onto the top of the brownie batter in an alternating, random pattern. Take a butter knife and gently make large, figure-eight motions through the dollops to create distinct ribbons and swirls. Avoid over-swirling to prevent the flavors from mixing completely.

Step 4: Bake and Cool
Place the pan in the oven and bake for 20 to 30 minutes. The brownies are done when the edges look set and a knife inserted into the center comes out with only moist crumbs attached. Allow the brownies to cool completely before slicing (about 2 hours) for the neatest, cleanest cuts.

Step 5: Serve
Once fully cooled, lift the brownies out of the pan using the parchment paper slings and slice them into 9 or 16 squares.