1 lb. cavatappi, large elbow macaroni, or fusilli
Kosher salt
2 cups whole milk
1 Tbsp. cornstarch
4 oz. cream cheese
2 Tbsp. Dijon mustard
12 oz. sharp cheddar, coarsely grated
4 oz. prepared finely grated Parmesan
Freshly ground black pepper
1 cup panko
Place a rack in middle of oven; preheat to 450°F. Cook 1 lb. cavatappi, large elbow macaroni, or fusilli in a large pot of boiling heavily salted water until very al dente, about half the time of package directions. Drain; reserve pot.
Whisk 2 cups whole milk and 1 Tbsp. cornstarch in a medium bowl to combine. Transfer to reserved pot. Whisk in 4 oz. cream cheese and 2 Tbsp. Dijon mustard and heat over medium, whisking often, until cream cheese is melted and mixture is just beginning to bubble, about 4 minutes. Gradually whisk in 12 oz. sharp cheddar, coarsely grated, and 4 oz. prepared finely grated Parmesan and cook, whisking constantly, until cheese is mostly melted, 1–2 minutes. Remove sauce from heat, add pasta, and mix until well coated. Season with kosher salt and lots of freshly ground black pepper.
Scrape pasta into an 18x13" rimmed baking sheet (no need to line or grease) and spread out to edges. Scatter 1 cup panko evenly over pasta and bake, rotating baking sheet halfway through, until panko is toasted and pasta is crisp and deeply browned around the edges, 25–30 minutes. Let cool slightly before serving.