¼ stick (two tablespoons) unsalted butter, melted 2 cups panko (Japanese bread crumbs) 1 cup coarsely grated extra-sharp Cheddar (about 4 ounces)
FOR CHEESE SAUCE AND MACARONI
3 tablespoons unsalted butter 3 tablespoons all-purpose flour ½ teaspoon red pepper flakes 2 ¾ cups whole milk ¾ cups heavy cream 4 cups coarsely grated extra-sharp Cheddar (about 1 pound) 2 teaspoons Dijon mustard 1 ½ teaspoons salt ¼ teaspoon freshly ground black pepper ¾ pound elbow macaroni Put a rack in the middle of the oven and preheat oven to 400 degrees F. Butter a 3-quart shallow baking dish.
MAKE THE TOPPING: Stir together butter, panko, and cheese in a bowl until well combined.
MAKE THE SAUCE: Melt butter in a 5-quart heavy pot over moderately low heat. Whisk in flour and red pepper flakes and cook, whisking, for 3 minutes to make a roux. Whisk in milk in a slow stream, then bring sauce to a boil, whisking constantly. Simmer, whisking occasionally, for 3 minutes. Stir in cream, Cheddar, mustard, salt, and pepper. Remove pot from heat and cover surface of sauce with wax paper to prevent a skin from forming.
COOK THE MACARONI AND ASSEMBLE THE DISH: Cook macaroni in a 6-quart pot of boiling salted water (1 tablespoon salt per every 4 quarts of water) until al dente. Reserve 1 cup pasta cooking water and drain macaroni.
Stir together macaroni, reserved cooking water, and sauce in a large bowl, the transfer to baking dish (mixture will be loose).
Sprinkle topping evenly over macaroni. Bake until top is golden and bubbling, 25 to 35 minutes.
COOK’S NOTE: The topping can be made up to 1 day ahead and refrigerated, covered.