Preheat oven to 350 degrees. Butter the bottom only of a 13x9 baking dish. In a large mixing bowl, combine cake mix, water, vegetable oil, eggs and mashed bananas. Using an electric mixer, beat ingredients on low speed for 30 seconds then increase speed to medium and beat for 2 minutes, scrapping down the sides of the bowl frequently.
Pour mixture into prepared baking dish and bake 33 - 38 minutes until toothpick inserted in the center comes out clean. Allow cake to cool completely about 1 hour.
In a large mixing bowl, whisk together sweetened condensed milk, coconut milk and half and half. Poke top of entire cake every 1/2 inch with a large rimmed fork. Pour milk mixture evenly over cake. Cover cake and refrigerate 2 hours (or overnight) until liquid is absorbed into cake.
In a medium mixing bowl, using an electric mixer, whip heavy cream until soft peaks form. Add in granulated sugar and beat until stiff peaks form. Frost cake just before serving with sweetened whipped cream and sprinkle cake evenly with toasted coconut. Garnish cake slices with sliced bananas. Store cake in refrigerator.
Recipe Source: adapted slightly from Betty Crocker