In a large mixing bowl, cream together the sugar and butter until well-blended. Add the maple syrup to the butter and sugar mixture and stir until incorporated. Mix the baking powder into the flour in a separate bowl to avoid pockets of baking powder in the finished cake. Alternate between adding the milk and the flour mixture to the butter mixture, stirring after each addition. Beat the egg whites using an electric mixer in a separate bowl until soft peaks form. Soft peaks will form when the beaters are pulled up out of the egg whites and will then curl over. After adding all of the milk and flour and stirring until the cake batter has a uniform color and texture, gently fold in the egg whites. Spray the pan with nonstick cooking spray and pour the cake batter into the pan. Bake the cake at 350 F for 45 to 60 minutes, or until a toothpick inserted in the center comes out clean.