Spray 9x13 casserole dish with cooking spray

Spread ham into the bottom of prepared casserole dish. Butter one side of each bread slice; cut slices into cubes. Arrange buttered bread cubes atop ham; layer American cheese over bread cubes

Whisk milk, eggs, mustard powder, and salt together in a bowl, pour over American cheese layer. Cover dish with aluminum foil and refrigerate 8 hours or overnight.

Preheat oven to 350 degrees F remove aluminum foil from casserole.

Bake in the preheated oven until cheese is melted and eggs are set in the middle, about 1 hour