Make day before serving
Combine pudding mix & milk; beat. Stir in Cool Whip.
Place a layer of graham crackers in bottom of 9x13 dish. Spread 1/2 filling mixture on top. Repeat with another layer of graham crackers, add rest of filling, top with final layer of graham crackers.
Bring cocoa, milk & butter to a rolling boil; a full minute, stirring frequently.
Remove from heat, cool 1 minute.
Add butter & vanilla, stir till butter is melted.
Pour choc topping over graham crackers & refrigerate overnight.