1. Prepare, bake, and cool brownie mix according to package directions for 13 x 9-inch baking pan.
2. With spoon, evenly drop marshmallow cream onto cooled brownies.
3. In small microwave-safe bowl, microwave frosting at HIGH (Full Power) 30 seconds or until melted and smooth. Repeat with peanut butter. Pour melted frosting, then melted peanut butter over marshmallow cream and swirl with butter knife to marble. Let cool completely. To serve, cut into squares.