Prepare the brownies according to bag directions. Deposit 3 pounds batter into a sprayed and papered half sheet tray. Bake at 350*F for 30 minutes or until completely set. Let cool completely. Melt 2oz butter and 11oz mini marshmallows, stir in 8oz puffed rice and spread over the cooled brownie half sheet. Let set up completely, drizzle with chocolate flat icing, and cut.