Combine dry pudding mix and w½ cups milk; beat until well mixed. Stir in Cool Whip.
Place a layer of graham crackers in the bottom of a 9x13” dish.
Spread ½ 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.
Prepare the chocolate topping
In a medium-sized saucepan, 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. Spread evenly to completely cover graham crackers.
Cover dessert and refrigerate over night.
Notes
Keep in mind when making this must chill overnight to set and allow the layers to meld.
When making the chocolate topping, be sure to let the mixture come to a full rolling boil. Otherwise, it will stay too soft and runny on top of the dessert — rather than setting into a nice fudgy layer.