In a large mixing bowl, whisk together the pudding mix and milk for about a minute to combine until smooth. Add in the peanut butter and whisk some more, until it's fully mixed in. Finally, fold in the cool whip until it's mixed, but don't over-mix or you'll knock it flat.
Line the bottom of a 9x13" or similar baking dish with graham crackers.
Evenly spread half the pudding filling over the layer of graham crackers. It's easy to manage so you can smooth it out with a spatula easily.
Add another layer of graham crackers, then spread the remaining pudding mix over the top, leveling it smooth. Line the top with a third and final layer of graham crackers. Set this aside.
In a saucepan over low heat, add the heavy cream, corn syrup, and chocolate chips. Heat and stir until the chocolate is melted and you've mixed it into a smooth ganache. It should take about five minutes, give or take.
Pour the chocolate over the top of the layered icebox cake.
Place the whole thing in the fridge to chill. I find it best to let it chill overnight, to help firm everything up. To serve, cut portions and scoop with a spatula. Or go to town with a serving spoon if you don't need it to be presentable.