Directions:

Prepare cake mix according to package directions for a 9x13 cake and bake.
Once cake comes out of the oven, allow it to cool for just a couple of minutes.
Then use a wooden spoon handle or some other similarly-sized object, begin poking holes in the cake. You want the holes to be fairly big so that the pudding has plenty of room to get down in there. Be sure to poke right down to the bottom of the cake.
In a bowl, whisk together instant pudding mix with 4 cups milk. Stir until all the lumps are gone. Let the pudding sit for just about 2 minutes, so it has just slightly begun to thicken but not fully set. It should still be easily pourable.
Pour pudding over cake, taking care to get it into the holes as much as possible. Spread it all out and use the back of the spoon to gently push pudding down into the holes.
Put the cake into the fridge to set and cool.
Once cake has completely cooled, spread on whipped topping. Spread crushed wafers on top of the cake. You can do this part immediately before serving the cake if you like.
This will ensure the wafers are crunchy when you serve it.
Serve with freshly sliced bananas and whole vanilla wafers, if desired. Keep refrigerated.