For the filling:

Combine pudding mix and milk; beat. Stir in Cool Whip.

Place a layer of graham crackers in the bottom of a 9×13″ dish. Spread 1/2 of the pudding mixture over the graham crackers. Repeat with another layer of graham crackers and pudding; then top with a final layer of graham crackers.

For the topping:

Bring cocoa, milk, and sugar to a full rolling boil; boil about 1 minute, stirring very frequently. Remove from heat and cool one minute. Add butter and vanilla; stir until butter is melted.

Pour chocolate topping over the top layer of graham crackers.

Refrigerate over night.