Prepare the Pudding Mixture:

In a large mixing bowl, whisk together the instant vanilla pudding mix and milk. Stir for about 2 minutes until the pudding thickens. Gently fold in the whipped topping until fully combined. Set aside.

Layer the Cake:

In a 9x13-inch baking dish, place a layer of graham crackers to cover the bottom of the dish. Spread half of the pudding mixture over the graham crackers. Add another layer of graham crackers on top of the pudding. Then, spread the remaining pudding mixture over the crackers, smoothing it into an even layer. Finish with a final layer of graham crackers on top.

Make the Chocolate Topping:

In a medium saucepan, melt the butter over medium heat. Stir in the milk and cocoa powder, and bring to a gentle simmer. Once it starts simmering, remove from heat and stir in the vanilla extract. Gradually whisk in the powdered sugar until the mixture is smooth.

Pour the Topping:

Pour the chocolate glaze over the top layer of graham crackers, spreading it evenly with a spatula.

Chill:

Cover the baking dish with plastic wrap or foil and refrigerate the cake for at least 4 hours, or overnight. This allows the graham crackers to soften and the flavors to meld together.

Serve:

Once chilled, slice into squares and serve! Enjoy the creamy, chocolaty goodness!