Preheat the oven to 350 °F. Grease the bottom of a 13x9-inch pan.
Combine the brownie mix, eggs, water and oil in a large bowl. Stir with a spoon until well blended, about 50 strokes. Spread the batter in the pan. Bake at 350 °F for 27 to 30 minutes or until set.
Spread the marshmallow cream gently over the hot brownies. If it doesn't spread evenly, return the baking pan to the oven with the oven off for 2-3 minutes and try again. Place the frosting in a microwave-safe bowl. Microwave on high power for 15 to 30 seconds. Stir until smooth. Pour the frosting over the marshmallow cream. Swirl with a knife to marbleize. Cool completely. Cut into bars.
