Make the pudding base: In a large mixing bowl, whisk together the vanilla pudding mix and whole milk for about 2 minutes until smooth and slightly thickened. Let it rest for 5 minutes. It will continue to firm as it sits.

Whip the cream: In a separate chilled bowl, beat the heavy whipping cream with a hand mixer or stand mixer until stiff peaks form. Do not rush this step — you want the peaks to stand straight up without curling over. This is what gives the filling its lightness.

Combine: Using a large rubber spatula, gently fold the whipped cream into the pudding mixture in wide, slow strokes. Work until just combined and uniformly light and fluffy. Refrigerate while you prepare the pan.

Build the layers: Arrange a single layer of graham crackers across the bottom of your 14x11-inch deep lasagna pan, breaking crackers as needed to fill in the edges. Spread half of the cream filling evenly over the crackers. Add a second layer of graham crackers. Spread the remaining half of the filling evenly. Add a third and final layer of graham crackers on top.

Make the chocolate topping: In a medium saucepan over medium heat, whisk together the milk, sugar, and cocoa powder until smooth and no lumps remain. Bring to a rolling boil, stirring frequently. Remove from heat. Add the butter and stir until fully melted and glossy. Stir in the vanilla extract. Let cool for 10 minutes.

Top and chill: Pour the chocolate topping evenly over the top layer of graham crackers and spread to the edges. Cover tightly with plastic wrap and refrigerate overnight — at least 8 hours. The graham crackers will absorb moisture from the filling and soften completely into a cake-like texture.

Slice and serve: Remove from the refrigerator and slice into squares. A sharp knife wiped clean between cuts gives you the cleanest slices. Serve cold, directly from the pan.