For the topping:
- 2 tablespoons unsalted butter, plus more for baking dish
- 2 tablespoons extra-virgin olive oil
- 2 cups panko (Japanese breadcrumbs)
- 2 large garlic cloves, minced
- 2 ounces finely grated Parmesan (about 1/2 cup)
- 1/2 teaspoon kosher salt
For the macaroni and sauce:
- 2 teaspoons kosher salt, plus more
- 1 pound dried elbow macaroni
- 1/2 cup (1 stick) unsalted butter
- 6 tablespoons all-purpose flour
- 4 cups whole milk
- 3/4 cup heavy cream
- 1/2 teaspoon ground white pepper
- 1 1/2 tablespoons mustard powder
- 16 ounces coarsely grated extra-sharp cheddar (about 6 cups)
- 2 ounces grated Parmesan (about 1/2 cup)
Boil the pasta: Set a large, covered pot of salted water over high heat to boil. Add macaroni and cook until just al dente (avoid overcooking). Drain macaroni and set aside.
While pasta is cooking, make the topping: In a large skillet over medium heat, heat butter and oil until butter foam subsides. Add panko and garlic; cook, stirring, until crumbs are golden brown, 4–6 minutes. Transfer crumbs to a medium bowl, stir in Parmesan and salt, and set aside.
Preheat oven to 400°F with rack in the middle of oven. Butter the baking dish.
Prepare the sauce: In a large wide pot over medium-low heat, melt the butter. Sprinkle flour over butter, whisking to incorporate and make a roux. Cook, stirring constantly with a wooden spoon, until roux is light golden, about 4 minutes. Gradually pour in milk and cream, whisking constantly to incorporate and make a béchamel sauce. Raise heat to medium-high and bring sauce to a low boil, whisking constantly. Reduce to a simmer, whisking occasionally, and cook until béchamel sauce is thick and coats the back of a spoon, about 3 minutes more.
Put it together: Add 2 tsp. salt, pepper, and mustard powder. Add the cheeses in three batches, whisking until each addition is completely melted before adding more. Remove from heat.
Add the drained macaroni to the pot with the cheese sauce and stir well to coat. Transfer macaroni mixture to the buttered baking dish and place on a rimmed baking sheet. Sprinkle topping evenly over macaroni and bake until golden and bubbling, 18–22 minutes. Let cool 15 minutes before serving.