Spread evenly in greased 9x13-inch baking pan. Bake according to box directions.
Pre-heat oven to 350°F. Mix together brownie mix, egg, apple sauce and water.(see amounts for your box mix below) The batter will be slightly lumpy, do not overmix.
Sprinkle marshmallows, nuts and chocolate chips on the top. Bake for an additional 3-5 minutes until marshmallows are just starting to brown. Cool completely and cut into bars.