Preheat oven to 325℉. Grease a 10 x 15 x 1 inch cookie sheet with nonstick cooking spray.
In a large bowl, beat together cake mix, water, oil and eggs until combined. Pour cake batter into prepared cookie sheet and bake for 18-20 minutes, or until toothpick inserted comes out clean.
Let cool completely.
In a medium sized bowl mix 2 cups milk with French vanilla pudding and whisk for 2 minutes. Set aside.
In another medium sized bowl mix 2 cups milk with the chocolate pudding and whisk for 2 minutes.
Combine prepared vanilla pudding and chocolate pudding in one large mixing bowl and spread on top of the cake.
Carefully spread Cool Whip on top of the pudding.
Use a grater or vegetable peeler to shave off pieces of the chocolate bar and sprinkle over the top of the cake.
Keep refrigerated until ready to serve.
Notes
We recommend refrigerating this cake and keeping it in a container, or tightly wrapped up with saran wrap or foil, for the best results. It will be good in the fridge for about 3 days, but recommended you eat the day of making it.