Preheat the oven to 200C/180C fan/Gas 6. Bring a large pan of salted water to the boil and cook the macaroni for 2 minutes less than the packet says, then drain.
Meanwhile, melt the butter in a saucepan over a medium heat, stir in the flour and cook for 1 minute to make a smooth paste.
Add the milk a splash at a time, stirring well after each addition until smooth, then pour in the rest and simmer for 3-4 minutes until you have a thick sauce.
Take the sauce off the heat and stir in the cheese, mustard and plenty of black pepper until melted. Taste and add a little salt if needed.
Stir the drained macaroni into the sauce, then tip everything into an ovenproof dish and scatter the breadcrumbs on top if using.
Bake for 15-20 minutes until golden and bubbling. Leave to stand for 5 minutes before serving.