In a large bowl, use an electric mixer on medium-high speed to combine the pudding mix and cold milk for 2 minutes or just until thickened. Set aside.
In a separate bowl, whip the heavy cream and vanilla extract until soft peaks form.
By hand using a large rubber spatula, fold the whipped cream gently into the pudding until smooth and airy. Stir gently to avoid overmixing.
In an 8x8-inch or 9x9-inch dish, arrange a single layer of graham crackers to fully cover the bottom, breaking pieces as needed.
Spread half of the vanilla pudding mixture evenly over the crackers.
Repeat with a second layer of graham crackers, then spread the remaining vanilla pudding on top.
Finish with a final layer of graham crackers, pressing gently to level the surface.
Make the Chocolate Topping: Place chocolate chips and heavy cream in a heat-safe bowl. Microwave in 20–30 second intervals, stirring between, until smooth and fully melted. Stir in butter, if using.
Pour the warm chocolate topping evenly over the top layer, spreading gently to the edges.
Cover and refrigerate for at least 6 hours, preferably overnight, until the graham crackers have softened and the layers are set.
Slice and serve chilled.
Notes
Whipped Cream - You can use one 8 ounce tub of Cool Whip or your favorite brand of whipped topping, in place of homemade whipped cream for the pudding mixture. Fold whipped cream gently to keep the filling light and airy.
Graham Crackers - You can use classic graham crackers or honey graham crackers for the "pastry" layer.
Allow Ample Chill Time - Overnight chilling creates the most cake-like texture, giving the graham crackers time to soften making this dessert ideal for making in advance.
Milk - I highly recommend using whole milk for a richer, more stable filling.
Chocolate - You can use milk chocolate chips or dark chocolate chips for the topping.
Slicing and Serving - The chocolate ganache topping will naturally become firm once this easy dessert is chilled. You can use a warm the knife between slices for clean edges.