Preheat your oven to 350℉ and lightly grease a 9x13 cake pan.
Add the cake mix, eggs, water and melted butter to a large mixing bowl and whisk together into a smooth batter.
Pour the batter into the cake pan and bake for 25 minutes. The cake will be golden brown and spring back when touched.
Let the cake cool in the pan for about 10 minutes then use the handle of a wooden spoon to poke holes across the whole surface of the cake, making holes that go to the bottom of the pan, spaced about 1 ½ inches apart.
Pour the banana pudding mix into a large bowl and add the milk. Whisk for about 2 minutes, making sure the pudding is nice and smooth. It will start to set and get thick almost immediately.
Pour the pudding over the cake. Try to get the pudding into the holes by using the back of a spoon or an offset spatula to spread the pudding and also push it down.
Put the cake in the fridge to cool and set for about 2 hours.
Remove the cake from the fridge when you are ready to serve it. Spread the Cool Whip topping over the cake then sprinkle the crushed vanilla wafers and banana pieces over the top of the Cool Whip. Slice and enjoy!
Notes
If you do not have a wooden spoon to make holes, you could also use a bubble tea straw or just your finger. You want holes that are about ¼ to ½ inch in size so the pudding can get into the cake easily.