Beat butter at medium speed with an electric mixer until creamy; gradually add sugar, beating well. Add eggs, 1 at a time, beating until blended. Stir in flavoring.
Combine flour, soda, and salt; gradually add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat until blended. Remove 2 1/2 cups batter.
Stir together 2 1/2 cups batter, cocoa, and vanilla; pour into 1 greased and floured 9-inch round cakepan.
Stir 1 cup banana into remaining batter; pour evenly into 2 greased and floured 9-inch round cakepans.
Bake layers at 350° for 20 to 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes. Remove from pans, and wrap in plastic wrap. Freeze 1 hour.
Spread 1 banana layer with Buttercream Frosting; arrange banana slices on top. Top with chocolate layer, and spread with frosting; top with strawberries. Top with remaining banana layer; spread top and sides with frosting. Garnish, if desired.