Step 1

Spray a 9x13-inch casserole dish with cooking spray.

Step 2

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.

Step 3

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.

Step 4

Preheat oven to 350 degrees F (175 degrees C). Remove aluminum foil from casserole.

Step 5

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