Preheat oven to 350°F. Line an 8x8 pan with foil, so that it's hanging over the sides. Spray with nonstick spray.

Prepare brownie batter according to directions on box. Spread batter in prepared pan. In a microwave-safe bowl, heat peanut butter for about 15 seconds. Spoon into batter, then swirl with a knife. Bake brownies for about 20 minutes or until a toothpick inserted in the center comes out with moist crumbs but no raw batter. Let pan cool completely on a wire rack.

Spoon marshmallow fluff on top of brownies and spread to edges. Place pan in the fridge for at least an hour, until marshmallow is cold and firm.

In a small saucepan, combine peanut butter, powdered sugar, corn syrup, and heavy cream. Stir over medium heat until smooth and combined. Pour over marshmallow fluff and spread to edges. Place pan back in the refrigerator until chilled and firm. Remove brownies from pan, peel back foil, and cut into squares.