Heat oven to 177°C (163°Cfor dark or nonstick pan). Grease bottom only of 33x23-cm pan.
In large bowl, beat cake mix, water, oil, eggs and mashed bananas with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Pour into pan.
Bake 33 to 38 minutes or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.
Poke top of cake every 1cmwith long-tined fork, wiping fork occasionally to reduce sticking. In large bowl, stir together condensed milk, coconut milk and whipping cream. Carefully pour evenly over top of cake. Cover; refrigerate at least 2 hours or overnight until mixture is absorbed into cake.
Spread frosting over cake. Garnish each serving with banana slices and/or toasted coconut. Store loosely covered in refrigerator.