Line bottom of 9x13 inch baking dish with single layer of graham crackers. Break crackers as needed to fit and cover entire bottom completely with no gaps. Some pieces will be smaller to fill edges - that's fine.

In large mixing bowl, whisk instant vanilla pudding mix with cold milk. Use ice-cold milk straight from refrigerator. Beat with electric mixer or whisk for 2 minutes until thick and smooth. Let set 5 minutes until pudding is firm and holds its shape. Fold in Cool Whip or whipped topping gently until completely combined. Mixture should be thick and creamy.

Spread half of pudding mixture evenly over graham cracker layer in pan. Use offset spatula or back of spoon to smooth into even layer reaching all edges. Place another layer of graham crackers over pudding covering completely. Spread remaining pudding mixture over second cracker layer smoothing to edges. Top with final layer of graham crackers covering all pudding. Press down gently so layers connect.

In medium saucepan over low heat or in microwave-safe bowl, melt butter completely. Don't let it brown or burn. Remove from heat. Whisk in cocoa powder until smooth with no lumps. Add powdered sugar and stir until combined. Mix in vanilla extract. Add milk one tablespoon at a time stirring until frosting reaches thick but pourable consistency. Frosting should be glossy and smooth.

While frosting is still warm, pour over top graham cracker layer. Spread evenly to all edges using offset spatula. Warm frosting spreads much easier than cold and creates smooth glossy finish. Work quickly before frosting cools and thickens. Make sure entire top is covered with even layer of chocolate.

Cover pan tightly with plastic wrap or aluminum foil. Refrigerate overnight or minimum 8 hours. This chilling time is absolutely essential - don't skip or reduce it. Graham crackers need this time to absorb moisture from pudding becoming soft and cake-like. Shorter chilling means crunchy crackers not tender cake texture.

Keep refrigerated until ready to serve. Cut into 12 squares using sharp knife. Wipe knife clean between cuts for neat slices. Graham crackers should be completely soft like cake not crunchy. Frosting should slice cleanly without cracking. Serve cold straight from refrigerator. Store leftovers covered in fridge up to 4 days.