To prepare cake, combine the first 7 ingredients; stir well with a whisk. Lightly spoon flour into a dry measuring cup, and level with a knife. Combine flour, 1 cup granulated sugar, and next 4 ingredients (1 cup granulated sugar through salt) in a large bowl. Add buttermilk mixture to flour mixture, stirring just until blended.
Beat egg whites with a mixer at high speed until soft peaks form. Add 1/2 cup granulated sugar, 1 tablespoon at a time, beating until stiff peaks form. Fold the beaten egg whites into batter; pour batter into a 13 x 9-inch baking pan coated with cooking spray.
Bake at 350° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool completely on a wire rack.
To prepare the icing, combine powdered sugar, fat-free milk, and 1/8 teaspoon banana extract (if desired). Fold in the whipped topping. Spread icing over cake; cover and chill at least 1 hour. Top with banana slices, and sprinkle with cocoa, if desired.