Add the pudding & milk to a large mixing bowl. Whisk them together until the mixture's smooth. Continue whisking until thickened, about 3 minutes.
Spray a 9x13" lightly with non stick cooking spray. Pour the pudding mixture into the casserole dish.
Layer the graham cracker's on top of the pudding mixture in a single layer. Add half of the Hershey's bars on top of the cracker layer.
Sprinkle the boxed cake mix evenly out over top of the cracker & candy bar layers. Add the butter slices evenly out over the top of the casserole.
Bake the cobbler at 350 degrees for 22-25 minutes.
Remove the cobbler from the oven, and using a sturdy spoon mix in any remaining patches of dry cake mix- breaking up the graham cracker pieces slightly as you do.
Layer the marshmallows evenly out over the top. Spread the remaining chocolates evenly out overtop of the marshmallows.
Return the cobbler to the oven, and broil it until the marshmallows are nicely browned and toasted.
Remove the casserole from the oven, scoop into bowls, and serve immediately.