In a large mixing bowl, whisk the instant vanilla pudding mix with the cold milk for about 2 minutes, until it starts to thicken. Let it sit for 3 to 5 minutes so it can firm up slightly.

Gently fold the thawed whipped topping into the pudding until the mixture looks smooth, light, and even. Do not overmix.

Place a single layer of graham crackers across the bottom of a 9x13-inch baking dish, breaking crackers as needed to mostly cover the base.

Spread half of the pudding mixture over the graham crackers in an even layer.

Add another layer of graham crackers, then spread the remaining pudding mixture on top. Finish with one last layer of graham crackers.

Warm the heavy cream in a small saucepan over low heat until hot but not boiling. Remove from the heat and add the chopped chocolate, butter, and corn syrup or maple syrup if using. Let sit for 1 minute, then stir until glossy and smooth.

Pour the chocolate topping over the final graham cracker layer and gently spread it evenly over the top.

Cover and refrigerate for at least 8 hours, preferably overnight, until the graham crackers soften into cake-like layers.

Cut into squares and serve cold. For cleaner slices, wipe the knife between cuts.