Prepare your cake mix and bake according to box instructions in a 9x13 pan.
Allow cake to cool for about 5 minutes.
Poke the cake all over with the handle of a wooden or plastic spoon.
Mix instant pudding with milk in a large bowl. Stir it until there are no lumps.
Pour the pudding carefully over the cake, try to get as much into the holes as possible.
Spread it all over the top, there will be an extra layer on top of the cake.
Put the cake in the fridge to cool for about 2 hours, or overnight.
When the cake is ready to serve spread sliced bananas over the top of the pudding.
Spread cool whip evenly over the top of bananas and pudding.
Place in the fridge for 1 hour, or serve immediately.
When serving place 1 Nilla wafer and some crumbs, and some banana slices onto each slice.