Prepare cake mix according to package directions, omitting ¼ cup water beat on low speed until moistened, add banana beat for 2 minutes more. Pour into a 13x9 pan, bake at 350 for 30-35 minutes or until toothpick inserted in center comes out clean

For frosting, heat butter and water in a saucepan until butter is melted; set aside. Ina mixing bowl combine 4 cups powdered sugar and cocoa; add butter mixture, banana and vanilla; beat until smooth add enough remaining sugar until frosting reaches desired spreading consistency