Preheat oven to 350 °F. Grease bottom of 13×9-inch pan.
Combine brownie mix, eggs, water and oil in large bowl. Stir with spoon until well blended, about 50 strokes. Spread in pan. Bake at 350 °F for 27 to 30 minutes or until set.
Spread marshmallow cream gently over hot brownies. Place frosting in microwave-safe bowl. Microwave on high power for 15 to 30 seconds. Stir until smooth. Pour frosting over marshmallow cream. Swirl with knife to marbleize. Cool completely. Cut into bars.