1. Combine pudding and milk in large bowl. Whisk until thickened to pudding consistency. Place in fridge for 5 minutes. Fold in Cool Whip until well combined.
2. Layer graham crackers in single layer in 9x13 baking dish. Top with 1/2 of pudding mixture. Repeat layers ending with graham crackers. Refrigerate.
3. In a saucepan, melt butter over medium heat. Stir in cocoa powder and sugar. Once sugar begins to melt, whisk in milk. Bring to boil and cook, whisking constantly, for 2-3 minutes. Remove from heat and whisk in vanilla extract. Pour over the top layer of graham crackers. Refrigerate for at least 1 hour or until set.