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, begin poking holes in the cake. You want the holes to be big enough 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.
Pour pudding over cake. Taking care to pour it right 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.
Crush the vanilla wafers. 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 before serving.
You can spread the crushed wafers immediately before serving the cake if you like. This will ensure the wafers are crunchy when you serve it. Keep refrigerated.