4 tablespoons unsalted butter, plus more to grease the baking dish
1/2 cup bread crumbs
12 ounces sharp or extra-sharp Cheddar, grated (about 4 cups total)
1/2 pound elbow macaroni
2 cups whole milk
1 1/2 tablespoons all-purpose flour
1 teaspoon dry mustard
1/2 teaspoon fine sea salt
1/4 teaspoon freshly ground black pepper
Preheat the oven to 350°F. Lightly butter a 9- by 9-inch baking dish.
In a small saucepan over moderate heat, melt 2 tablespoons of the butter. Place the bread crumbs in a small bowl, drizzle with the melted butter, and toss to combine. Add a handful of the grated cheese to the bowl, mix well and set aside.
In a large pot of boiling salted water, cook the elbow macaroni until al dente. Drain the noodles, without rinsing, then return them to the pot they were cooked in.
In a medium saucepan over moderate heat, melt the remaining 2 tablespoons of butter. Add the flour and use a whisk to stir the mixture together, just until it starts to turn a light brown color, about 3 minutes. Slowly add the milk and whisk constantly until the sauce thickens, about 5 minutes. Remove from the heat and stir in the dry mustard, salt, and pepper.
Add the remaining grated cheese and stir with a spoon until it's completely melted. Adjust the seasoning to taste. Pour the cheese sauce over the cooked elbow macaroni and gently stir to coat the noodles in the sauce.
Pour the macaroni and cheese into the prepared baking dish and top with the breadcrumb-cheese mixture. Bake until light golden brown and bubbling, about 25-30 minutes. Serve hot.
DO AHEAD: Macaroni and cheese can be made ahead, put into the baking dish, cooled completely, and kept, covered in the refrigerator, for 2 days. Do not add the bread crumbs until ready to bake. (Chilled Macaroni and Cheese may take longer to be fully cooked.)