In a large bowl, whisk the instant vanilla pudding mix and cold milk until thickened (about 2 minutes).
In another bowl, whip the heavy cream with powdered sugar (if using) until stiff peaks form. Fold the whipped cream into the pudding mixture until smooth and fluffy.
Assemble the Cake:
In a 9×13-inch baking dish, lay a single layer of graham crackers, covering the bottom completely.
Spread half of the pudding mixture over the graham crackers evenly.
Add another layer of graham crackers, then the remaining pudding mixture.
Top with a final layer of graham crackers.
Prepare the Chocolate Topping:
In a small saucepan, combine cocoa powder, granulated sugar, milk, and butter over medium heat. Stir constantly until the butter melts and the mixture becomes smooth and glossy.
Remove from heat and stir in vanilla extract. Let the chocolate topping cool for 5-10 minutes.
Finish the Cake:
Pour the chocolate topping over the final layer of graham crackers, spreading evenly with a spatula.
Chill:
Cover the dish and refrigerate for at least 4 hours (or overnight) to allow the layers to set and the graham crackers to soften.
Serve:
Slice into squares and serve chilled. Enjoy this creamy, chocolatey delight!
