8 ounces dried jumbo elbow macaroni, conchiglie, or pipette pasta
4 tablespoons unsalted butter
1 cup panko or other breadcrumbs
1 teaspoon coarsely chopped fresh thyme leaves
3 tablespoons all-purpose flour
1 1/2 cups whole milk
1 1/2 cups grated sharp cheddar cheese
1 teaspoon Dijon mustard
1/4 teaspoon kosher salt
1/8 teaspoon paprika
Arrange a rack in the middle of the oven and heat to 400°F. Meanwhile, bring a large pot of salted water to a boil. Add the pasta and cook per package instructions until al dente. Drain and set aside.
Melt the butter in a 9- or 10-inch cast iron skillet over medium heat. Remove 1 tablespoon of the butter and combine with the breadcrumbs and thyme in a small bowl; set aside.
Sprinkle the flour evenly into the skillet and whisk until fragrant, 1 to 2 minutes. Slowly add the milk while whisking to ensure it doesn't clump. Switch to a wooden spoon and continue cooking the sauce, stirring constantly, until it has noticeably thickened and coats the back of the spoon, 2 to 3 minutes.
Remove the skillet from the heat and stir the cheese into the sauce, one handful at a time, until melted and smooth. Stir in mustard, salt, and paprika. Add the cooked pasta and stir gently until the pasta and sauce are evenly combined. Sprinkle the breadcrumbs evenly over top.
Transfer the skillet to the oven and bake until the sauce is bubbling and the top is golden-brown, 10 to 15 minutes.