Place 6 bread slices in a buttered 9 x 13-inch baking dish. Layer ham, cheese and mushrooms on bread. Cover with remaining slices of bread. Combine eggs, milk and salt, beating well. Pour over bread and refrigerate overnight, covered. Top with croutons and dot with butter. Bake, uncovered, at 350° for 1 hour and 15 minutes. Let stand 10 minutes before cutting.