In a large bowl, whisk together the instant vanilla pudding mix and cold milk for about 2 minutes until thickened. Set aside.
In a separate bowl, combine the heavy cream, powdered sugar, and vanilla. Using a hand or stand mixer, whip until stiff peaks form.
Add the whipped cream to the pudding mixture and gently fold together until smooth and fully combined.
In a shallow dish, combine the cold milk and vanilla for the graham cracker dip. Stir briefly to combine and set aside.
Grab a 9x13-inch dish. Working with one graham cracker at a time, quickly dip each cracker into the milk mixture (see note 2), and arrange them in an even layer across the bottom of the dish. Break crackers as needed to fit the pan.
Spread half of the pudding mixture evenly over the graham cracker layer.
Add a second layer of dipped graham crackers, then spread the remaining pudding mixture evenly over the top. Finish with a final layer of dipped graham crackers.
Remove the ganache from the refrigerator and give it a quick stir. It should still be smooth and pourable. Pour the ganache over the top layer of graham crackers and gently swirl the dish so the chocolate spreads evenly to the edges.
Cover the dish with plastic wrap and refrigerate for at least 3 hours to allow the filling to set and the graham crackers to soften.
When ready to serve, slice with a knife dipped in hot water and wiped dry for clean, tidy slices. Serve chilled.