1. Heat the oven to 200°C / 180°C fan / gas 6. Cook the macaroni in salted water for 2 minutes less than the packet suggests, then drain.
2. Melt the butter in a saucepan, stir in the flour and cook for 1 minute. Add the milk a little at a time, whisking until smooth. Simmer gently for 3–4 minutes, stirring, until it coats a spoon.
3. Take the pan off the heat. Stir in the Cheddar and 50 g Parmesan, then season with pepper and a little salt. Fold in the pasta. The sauce should still look slightly runny.
4. Spoon into a baking dish. Mix the breadcrumbs with the remaining Parmesan and scatter over the top. Bake for 15–18 minutes until bubbling and golden.
5. Rest for 3 minutes, then dust with 1 tsp black truffle powder. Taste a forkful and add more only if you want a stronger finish. Scatter over the parsley and serve.
