1. Stir together flour, baking powder and salt. Set aside.
2. In large bowl, gradually add sugar to egg, beating well after each addition. Stir in vanilla. Add KELLOGG'S CORN FLAKES® cereal and flour mixture. Mix well. Fold in walnuts and apples. Spread in 8 x 8 x 2-inch square pan coated with cooking spray.
3. Bake at 350° F about 30 minutes. Cut into wedges and serve warm or cold with whipped topping, if desired.