Boil macaroni in salted water until tender.

Drain the macaroni thoroughly.

In a bowl, stir in the beaten egg and butter with the macaroni.

In a separate small bowl, mix the dry mustard, a little salt, and pepper with a tablespoon of hot water to form a paste.

Add the milk to the mustard mixture.

Add most of the grated cheese to the macaroni mixture, reserving a small amount for topping.

Pour the macaroni mixture into a buttered casserole dish.

Pour the milk and mustard mixture over the macaroni.

Sprinkle the reserved cheese over the top.

Bake in a preheated oven at 350°F (175°C) for approximately 45 minutes, or until the custard is set and the top is crusty and golden brown.