Preheat oven to 180°C. Grease 23cm round loose-based cake pan or spring form pan at least 7cm deep with butter; lightly sprinkle with flour.
Beat cake mix, eggs, oil and water in large bowl with electric mixer on low speed until moistened. Beat on medium speed 2 minutes. Fold in mashed bananas with large spoon; pour batter into pan.
Bake 45 to 50 minutes or until toothpick inserted in center of cake comes out clean. Place coconut flakes in small pan, and toast in oven for last 5 minutes of bake time. Cool cakes in pans 10 minutes; remove to cooling rack to cool completely.
Place 50g of the cooled coconut flakes into food processor; cover and process until finely ground.
Beat whipping cream mix and cold milk in medium bowl as directed on package. Add finely ground coconut; beat until combined.
Cut cake in half horizontally. Place one half of the cake on serving plate cut side up. Spread half of the coconut cream over top. Place remaining half of cake, cut side down on top of coconut cream. Spread remaining coconut cream on top of cake. Decorate with mango and remaining coconut flakes. Cut into slices to serve.