Gather all ingredients. Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch baking dish.

Whisk cake mix, water, vegetable oil, and eggs together in a bowl until smooth; pour into the prepared baking dish.

Bake in the preheated oven until a toothpick inserted in the center of the cake comes out clean, 23 to 28 minutes.

Whisk cold milk and pudding mix together in a bowl until pudding is soft and thickened, about 2 minutes.

Poke holes into the cake using the handle of a clean wooden spoon. Pour pudding over cake, spreading evenly.

Spread whipped topping over pudding layer and sprinkle vanilla wafers over whipped topping.