Pour 6 cups of cold whole milk into a large bowl. Add both instant vanilla pudding mixes. Whisk briskly for 2-3 minutes until the mixture thickens. Gently fold in the thawed whipped topping and 1 teaspoon vanilla until smooth. Set aside.

Arrange a single, even layer of graham crackers to cover the bottom of a 9x13-inch baking dish. Spread half of the pudding mixture evenly over the crackers. Add a second full layer of graham crackers. Spread the remaining pudding on top. Finish with a final graham cracker layer.

Place the chocolate chips, 6 tablespoons unsalted butter, and 2 tablespoons whole milk in a small saucepan. Warm over low heat, stirring constantly, until smooth and glossy. Remove from the heat, stir in a pinch of salt and the remaining 1 teaspoon vanilla. Let the ganache cool a few minutes so it’s pourable but not scalding.

Pour the ganache over the top layer of graham crackers and spread evenly with an offset spatula or the back of a spoon. Cover the dish with plastic wrap. Refrigerate for at least 6-8 hours, preferably overnight so the crackers soften into cake-like layers.

Run a sharp knife under hot water, dry it, and slice the chilled cake into squares for cleaner edges. Wipe the knife between cuts for neat slices.