Instructions:
Make the pudding mixture: Whisk together pudding mix and cold milk until thickened. Gently fold in whipped topping.
Assemble the layers: In a 9×13-inch baking dish, arrange a layer of graham crackers. Spread half the pudding mixture over them. Add another layer of graham crackers and spread the remaining pudding mixture. Finish with a final layer of graham crackers.
Make the chocolate topping: Combine cocoa powder, sugar, melted butter, and milk in a saucepan. Cook over medium heat until boiling. Boil for 1 minute. Remove from heat and stir in vanilla extract. Let cool for 10-15 minutes until thickened.
Add the chocolate topping: Pour cooled chocolate topping over the top layer of graham crackers.
Chill the cake: Cover with plastic wrap and refrigerate for at least 4 hours, or overnight.
Serve: Slice and serve chilled.
