Prepare cake in 9 x 13 pan according to directions on box. When golden, remove from oven and let cool for a few minutes.
When just slightly cooled, use the handle end of a wooden spoon to poke holes into the baked cake. Don’t be shy; make the holes big enough for the pudding to seep into and go deep enough with the spoon that you hit the bottom of the pan.
Next, prepare pudding mixture. Combine 4 cups of milk and pudding mix; whisk until fully combined and there are no lumps.
Pour pudding mixture over entire cake. Spread evenly, filling all holes.
Refrigerate for two hours or until set.
Top with sliced banana, whipped topping, and crushed vanilla wafers! Serve and enjoy. (Refrigerate any leftovers.)