Stir together flour, baking powder and salt. Set Aside. In medium mixing bowl, beat together sugars, margarine and egg whites until well blended. Stir in flour mixture only until combined. Stir in cereal. Spread evenly into 9x9x2" baking pan coated with cooking spray. Bake at 350°F about 28 minutes or until lightly browned. Let cool completely before cutting. Store in airtight container