Lightly butter an 8-inch square baking dish and arrange 4 slices of the bread flat on the bottom. Cover each with equal amounts of cheese and ham, if desired. Top with the remaining bread.
Beat the eggs lightly in a small bowl and pour in the milk. Season with the salt and blend well. Pour over the sandwiches, cover and refrigerate for 1 hour.
Preheat the oven to 350 degrees F. Bake the sandwiches, uncovered, until lightly browned around the edges and set in the center, 40 to 45 minutes. Serve hot.
Recipe courtesy of Sara Moulton, Sara Moulton Cooks at Home, Broadway Books, 2002