2 cups diced cooked ham
2 tablespoons butter, or as needed
7 slices bread, crusts removed
1/2 pound American cheese, cubed
3 cups milk
6 eggs, beaten
3/4 teaspoon dry mustard
3/4 teaspoon salt
Spray a 9x13-inch 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 (175 degrees C). Remove aluminum foil from casserole. Bake in the preheated oven until cheese is melted and eggs are set in the middle, about 1 hour.