1. Preheat oven to 350°F. Spray 9-inch round cake pan with nonstick cooking spray.
2. To prepare Cake, in medium bowl, combine flour, baking powder, salt and baking soda.
3. In large bowl, beat granulated sugar and margarine with electric mixer at medium speed until blended. Whisk in apple sauce, 1/2 cup milk, egg whites and 1 teaspoon vanilla.
4. Add flour mixture to apple sauce mixture; stir until well blended. Pour batter into prepared pan.
5. Bake 35 to 40 minutes or until toothpick inserted in center comes out clean. Cool completely on wire rack. Split cake horizontally in half to make 2 layers.
6. To prepare Filling, prepare pudding mix according to package directions using 1-1/2 cups skim milk. (Or, substitute 1-1/2 cups prepared fat-free vanilla pudding for Filling.)
7. To prepare Chocolate Glaze, in small bowl, sift together powdered sugar and cocoa. Stir in 1 tablespoon milk and 1/2 teaspoon vanilla. Add water, 1 teaspoon at a time, until of desired spreading consistency. Place one cake layer on serving plate. Spread filling over cake. Top with second cake layer. Spread top of cake with Chocolate Glaze. Let stand until set. Garnish with lemon peel, if desired. Cut into 10 slices. Refrigerate leftovers.