For the macaroni: Preheat the oven to 350 degrees F. Butter a 2-quart casserole dish.
Bring 4 quarts of water to a boil in a large saucepan. Add the salt and macaroni. Bring the water back to a boil and cook the macaroni until tender, about 12 minutes. Drain well.
For the cheese sauce: Meanwhile, melt the butter in a 1-quart saucepan. Using a wire whisk, stir in the flour and salt, stirring and cooking over medium heat until the roux bubbles and the flour turns pale brown, about 3 minutes. Slowly whisk in 1 cup of the milk and then whisk in the remaining 1 cup milk. Continue to cook, stirring constantly, until the sauce thickens. Add the cheese and stir until it melts. Add the drained macaroni to the cheese sauce and mix thoroughly.
For the topping: In a small bowl, stir the breadcrumbs with the butter until the crumbs are moistened.
Transfer the macaroni and cheese to the prepared baking dish and top with the buttered breadcrumbs. Bake until the dish bubbles around the edges, about 15 minutes.