Heat oven to 350ºF. Grease a 4-quart casserole or souffle dish and set aside.
Place dry pasta and 1 tablespoon of the kosher salt in a 4-quart pot and barely cover with cold water. Bring just to a boil over high heat, stirring occasionally, then test for doneness. You're looking for al dente, that is, the noodles should put up some resistance to the tooth but not so much that it gums up in your back teeth, 9 to 10 minutes. Drain pasta in a colander but do not rinse.
Meanwhile, melt the butter in a 3-quart saucepan over medium heat. Whisk in the flour and cook, stirring occasionally, until the mixture is pale blond, about 3 minutes. Whisk in the mustard and paprika, and cook until aromatic, about another minute. Whisk in the milk, increase the heat to medium-high, and cook, whisking often, until mixture is slightly thickened, 7 to 8 minutes. Remove from heat. Stir in the cheese, remaining salt, and pepper. Fold in the macaroni and move to the prepared dish.
Sprinkle with the remaining cheese, then toss the breadcrumbs with the melted butter and distribute evenly over the top. Bake until golden on top, but not dried out, 15 to 20 minutes. Cool for 5 minutes, then serve to a grateful world.