Heat the oven to 375°F and butter a 2-quart baking dish. Boil macaroni in a large pot of water with the tablespoon salt until 1 minute short of the package time; drain.
While the pasta cooks, melt the 3 tablespoons butter in a saucepan over medium heat. Whisk in flour for 1 minute, then gradually whisk in milk.
Add mustard, sauce salt and pepper and simmer for 4 minutes until lightly thickened. Remove from the heat and stir in 8 ounces cheddar until smooth.
Fold in the drained pasta, pour into the dish and scatter the remaining cheddar over the surface.
Bake for 25 to 30 minutes until bubbling and golden on top. Rest for 10 minutes before serving.