Preheat the oven to 350° F. Grease a 9x13-inch baking dish and set aside.
In a large bowl, whisk together the cake mix and the packet of instant banana pudding. Using a soft silicone spatula stir in the eggs, water, and vegetable oil.
Fold 1 cup of mashed bananas into the cake batter until well combined and transfer the mixture to the prepared baking dish.
Bake for 35-40 minutes or until a toothpick inserted comes out clean. Allow to cool for 20 minutes on a wire rack.
For the pudding:
In a large bowl, combine 1 cup of the mashed banana and milk. Sprinkle in the remaining two packets of banana pudding. Whisk the mixture until it starts to thicken. (It’s okay if the pudding is not totally smooth). Set aside.
Poke several large holes into the cooled cake using the end of a wooden spoon. (The larger the holes the more pudding you will get per bite).
Spoon the pudding mixture into the holes, concentrating on filling them until they can no longer hold any more pudding.
Spread any remaining pudding mixture over the cake making sure to get into all the corners. Let the cake sit in the refrigerator for at least 1 hour to allow the pudding to set.
Top with the whipped topping, using an offset spatula to decorate the top with peaks and valleys.
Scatter with broken Nilla wafer pieces and top with sliced bananas.
Cut and serve immediately.
Notes
Make sure the mashed bananas are free of any banana chunks (to give the cake and pudding a smooth texture).
Add the Nilla wafers and banana slices just before serving so the wafers stay crisp.
When adding the instant pudding, you are only adding the powder from the package; do not follow the instructions on the box to make the pudding.
This nutrition information is based on the exact products I used in this recipe. Brands and sizes of products could alter exact nutrition and should always be calculated independently.