For the cake: Preheat the oven to 350 degrees F. Grease only the bottom of a Pyrex 9X13 inch Deep Baking Dish. Set out a large mixing bowl. Pour the cake mix into the bowl. Add the mashed bananas, eggs, oil, and water. Whisk well until the mixture is mostly smooth, with just small chunks of banana left.
Pour the cake batter into the Pyrex dish. Bake undisturbed for 30 minutes. Check the cake by inserting a toothpick into the center. If it comes out clean, the cake is perfect. Cool the cake completely before layering the top.
For the Layers: While the cake is cooling, prep the other layers. Pour the instant pudding into the bowl of an electric stand mixer. Add the milk and beat for 2 minutes, as directed on the box. (FYI, You are using slightly less milk than mentioned on the box to create a thicker pudding layer.)
Once the cake is cool, spread the pudding over the top in an even layer. Cover the top of the pudding with banana slices.
Wipe out the stand mixer bowl and add in the heavy cream and sugar. Using the whip attachment, whip the cream on high until it forms firm peaks. (That means whip until the whipped cream is firm enough to stay in the position you place it in.)
Spread the whipped cream over the bananas, making sure to cover them completely. Then sprinkle the top with crushed vanilla wafers. Cover the dish and refrigerate until ready to serve.
If you plan to serve the cake within the next 30 minutes, you can decorate the top with fresh banana slices as well! However, if you are making this cake ahead of time, either add the banana slices right before serving or skip the bananas on top altogether.