Preheat the oven to 180°C (160°C fan forced). Grease a 33cm x 22cm rectangular cake pan and line the base and sides with baking paper.
Open the cake box, remove enclosed frosting sachet and reserve for another use. Place the cake mix, eggs, butter and milk in a bowl, then whisk (by hand or using an electric mixer) for 2-3 minutes until smooth and creamy. Whisk in the mashed bananas. Pour into the prepared pan and gently level the surface. Bake for 25-27 minutes until risen and golden and a skewer inserted into the cake comes out clean. Remove from the oven and cool in the pan for 5 minutes.
Using a metal skewer, poke holes in the cake at 1cm intervals, wiping the skewer clean occasionally to prevent sticking. For the tres leches, mix the condensed milk, coconut milk and cream together in a bowl. Setting aside 150ml of the mixture, spoon the remainder over the warm cake, spreading back and forth with the back of a spoon until absorbed into the holes. Leave in the pan to cool completely; about 1 1/2 hours.
To serve, remove the cake from the pan using the baking paper. Slice into 15 servings (see tip). Place each piece on a serving plate and drizzle over 1 teaspoon of the reserved milk mixture. Top each serving with a spoonful of frosting, a couple of banana slices and a sprinkling of toasted coconut.