This recipe was saved many years ago from the Food section of the Los Angeles Times. Unfortunately, I didn't also save the baking instructions, but you can figure that out by comparing with similar recipes.
Cook macaroni in boiling water until tender and drain thoroughly. Stir in butter and egg. Mix salt and mustard with hot water. Add to milk. Add cheese, reserving enough to sprinkle over top. Pour macaroni into buttered casserole. Add milk-cheese mixture. Sprinkle with remaining cheese.