Preheat oven at 375. Prepare a 9" baking pan with cooking spray and flour. = To prepare batter, combine flour, brown sugar, baking powder, baking soda, cinnamon, and salt in a mixing bowl. In another mixing bowl, combine yogurt, banana, applesauce, egg whites, and vanilla. Mix dry ingredients with wet ingredients just until combined. Pour batter into prepared pan. Bake 20 to 30 minutes or until cake springs back when lightly touched. Meanwhile, to prepare frosting, combine powdered sugar, margarine, and milk in a mixing bowl; beat until smooth. Spread frosting over cooled cake.