Preheat the oven to 350°F and spray a 9×13 inch pan with cooking spray.
In a large bowl, combine the cake mix, mashed bananas, water, vegetable oil, and eggs. Beat for 2 minutes until well combined.
Spread the cake batter evenly into the prepared pan and bake for 25 minutes. Test for doneness by inserting a toothpick into the center of the cake; if it comes out clean, the cake is done. Allow the cake to cool for 5 minutes.
In another bowl, whisk together the instant banana pudding mix, milk, and heavy whipping cream until smooth.
Use the handle of a wooden spoon or a knife to poke holes all over the cake. Pour some of the prepared pudding mixture over the cake, ensuring it fills the holes. Spread the remaining pudding mixture over the cake.
Cover the cake with plastic wrap and refrigerate for 2 hours to set.
Once chilled, arrange sliced bananas over the pudding layer, then spread the thawed Cool Whip over the bananas.
Crush 4 or 5 vanilla wafers and sprinkle them over the top of the cake. Cut the cake into 20 slices and serve, topping each slice with additional banana slices and a vanilla wafer.