For the filling:

1. Combine pudding mix and milk; beat. Fold in Cool Whip and mix.

2. Place a layer of graham crackers in the bottom of a 9x13" 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:

3. Bring cocoa, evaporated 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.

4. Pour chocolate topping over the top layer of graham crackers.

5. Refrigerate at least 4 hours (over night is best).

Source: BigOven