Prepare cake mix according to package directions for a 9x13 cake. Once cake comes out of the oven, allow it to cool for just a couple of minutes.
Then, with 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, prepare pudding. Whisk together instant pudding mix with 4 cups milk. Stir until all the lumps are gone.
Let 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 using the back of the spoon, gently push pudding down into the holes.
Put the cake into the fridge to set and cool (about 2 hours).
Once your cake has completely cooled, spread on whipped topping.
If you haven't done so already, crush your vanilla wafers. I just place mine in a ziploc bag and crush them with a rolling pin. Leave some of the pieces big. It's nice to have a bit of a crunch when you eat the cake.
Spread crushed wafers onto the 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 on top of each piece of cake. Keep refrigerated.