Heat oven to 180ºC Grease and flour 13x9 inch pan. In large bowl, combine cake mix, bananas, water, oil and eggs. Beat at low speed until moistened; beat 2 minutes at highest speed. Pour batter into prepared pan. Bake for 30-40 minutes or until tested done. Cool completely. In small bowl, combine frosting and mashed bananas. Spread over cooled cake. In small bowl, combine melted chocolate and powdered sugar. Add water until desired consistency, blending until smooth. Drizzle over frosting. Store in refrigerator.