1) Lightly grease a small (7- to 8-cup) casserole dish, for nicest presentation; or a 9" x 9" square cake pan.
2) Cut the bread into small (1/2" to 3/4" cubes). Place half the cubes into the prepared baking dish.
3) Layer the ham over the bread; sprinkle the cheese over the ham.
4) Top with the remaining bread.
5) In a separate bowl, whisk together the eggs, milk, salt, mustard, and cheese powder, if you're using it. Pour over the bread and ham in the dish, pushing the bread down into the milk.
6) Cover the dish, and refrigerate overnight.
7) Next day, preheat the oven to 375°F. Remove the casserole from the refrigerator.
8) Combine the bread crumbs and melted butter. Sprinkle evenly atop the casserole.
9) Bake the casserole for 40 to 45 minutes, or until a cake tester or paring knife inserted into the center comes out clean.
10) Remove the casserole from the oven. Serve hot or warm. Fruit salad is a tasty accompaniment.
11) Yield: 6 to 9 servings.