Heat oven to 350 degrees. Grease 13 x 9 inch pan. In large bowl combine brownie mix, water, oil and egg; beat 50 strokes by hand. Stir in mashed banana and chocolate chips. Spread in greased pan. Bake at 350 degrees for 28 to 35 minutes. DO NOT OVERBAKE. Cool completely. In medium bowl, combine topping ingredients; beat 1 minute or until thickened. Spread over cooled brownies. Garnish with banana slices dipped in lemon juices, if desired. Cut into bars. Store in refrigerator.