1. Preheat oven to 400°F. Spray 8- x 8-inch baking pan with cooking spray. Bring maple syrup just to a boil in small saucepan over medium-high heat. Pour evenly into pan. Carefully arrange apple slices in single layer over syrup; set aside.
2. Beat butter, sugar and egg in medium bowl with electric mixer on medium speed until well blended. Combine flour, baking powder and salt in small bowl. Add to sugar mixture alternately with the milk, beating on low speed after each addition just until blended. Spoon batter evenly over apples.
3. Bake 25 to 30 minutes, or until wooden pick inserted in center of cake comes out clean. Immediately invert dessert onto serving plate; remove pan. Cool slightly. Cut into 9 squares; top each with a serving of Reddi-wip just before serving.