Preheat oven to 350 ºF. Grease and flour two 9-inch round cake pans.
Combine cake mix, water, eggs and oil in large bowl. Beat at low speed with electric mixer until moistened. Beat at medium speed 2 minutes. Stir in bananas. Pour into prepared pans.
Bake 28 to 31 minutes or until toothpick inserted in center comes out clean. Cool in pans 15 minutes. Remove from pans; cool completely.
Fill and frost cake with frosting. Garnish as desired.