Preheat oven to 350. 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 into centers comes out clean. Cool in pans 15 mins. Remove from pans; cool completely. Frost
Frosting-Place the butter, cocoa powder, and chocolate syrup in a large mixing bowl. Blend with an electric mixer on low speed until just combined, 30 seconds. Stop the mixer and add confectioners sugar, milk, and vanilla. Blend with the mixer on low speed until the sugar is incorporated, 1 minute. Increase the speed to medium and beat until fluffy, 1 minute more. Frost your layers and sides and serve.