Prepare the filling:
- In a large bowl, whisk together the pudding mix and cold milk until thickened (about 2 minutes).
- Gently fold in the whipped topping until fully combined.
2. Layer the dessert:
- In a 9×13-inch baking dish, arrange a single layer of graham crackers to cover the bottom. Break crackers as needed to fit.
- Spread half of the pudding mixture evenly over the graham crackers.
- Add another layer of graham crackers, followed by the remaining pudding mixture.
- Top with a final layer of graham crackers.
3. Prepare the chocolate topping:
- In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer. Remove from heat and pour over the chopped chocolate in a bowl.
- Let sit for 2 minutes, then stir until smooth and glossy. Stir in the butter and corn syrup (if using) until fully incorporated.
4. Assemble the cake:
- Pour the chocolate topping over the top layer of graham crackers and spread evenly with a spatula.
5. Chill and serve:
- Cover the dish and refrigerate for at least 4 hours, or overnight, to allow the layers to soften and the flavors to meld.
- Slice into squares and serve chilled.