Preheat the oven to broil. Butter a baking dish. Melt the butter in a large saucepan over medium heat. Add the flour, and cook for one minute while stirring. Gradually add the milk, beating well after each addition to avoid lumps. Add the salt, pepper and mustard powder, then the grated cheese. Cook until the cheese has melted, then add in the cooked pasta and mix together.
Transfer the macaroni to the oven dish and top with the remaining handful of grated cheese. Place in oven for 5 to 10 minutes until golden brown and bubbling. Sprinkle with chopped chives and serve.