Prepare the yellow cake mix and bake in a 9x13 cake pan. Once it cools for 5 minutes, use the end of a whisk or wooden spoon to poke holes in straight lines in the cake.
Make the 2 boxes of pudding (using 4 cups of milk) making sure the lumps are gone. After 1-2 minutes, pour the pudding all over the top of the cake. Use a spatula and smooth it out so that the pudding falls into the holes. Spread evenly.
Put the cake in the refrigerator to completely cool for 30 minutes to an hour.
Spread cool whip over the whole cake.
Crush up the nilla wafers in a baggie (we used a rolling pin) and pour over the cake when you are about to serve. If you do it prior, they won't be as crunchy.