Melt 2 Tbsp. unsalted butter in a large skillet on the stovetop over medium heat. Add 2 Tbsp. extra-virgin olive oil, stirring, until butter foam subsides. Add 2 cups panko and 2 large garlic cloves, finely chopped; cook, stirring, until crumbs are golden brown, 4–6 minutes. Transfer crumbs to a medium bowl, stir in 2 oz. finely grated Parmesan and ½ tsp. Diamond Crystal or ¼ tsp. plus ⅛ tsp. Morton kosher salt, and set aside.

Bring a large pot of salted water, covered, to a boil over high heat. Add 1 lb. dried elbow macaroni and cook, uncovered, until just al dente (avoid overcooking). Drain par-cooked macaroni and set aside (do not rinse).

Place rack in middle of oven; preheat oven to 400°F. Grease a 13x9” baking dish with unsalted butter.

Melt ½ cup (1 stick) unsalted butter in a large wide pot over medium-low heat. Sprinkle 6 Tbsp. all-purpose 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 4 cups whole milk and ¾ cup heavy 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.

Add 2 tsp. Diamond Crystal or 1 tsp. Morton kosher salt, ½ tsp. ground white pepper, and 1½ Tbsp. mustard powder. Add 16 oz. coarsely grated extra-sharp cheddar cheese and 2 oz. grated Parmesan in three batches, whisking until each addition is melted before adding more. Remove from heat.

Add drained macaroni to pot with cheese sauce and stir well to coat. Transfer macaroni mixture to prepared baking dish and place on a rimmed baking sheet. Sprinkle panko topping evenly over macaroni and bake until golden and bubbling, 18–22 minutes. Let baked mac and cheese cool 15 minutes before serving.