Cook the Pasta: Boil macaroni in salted water until al dente. Drain and set aside.
Make the Sauce: Melt butter, add flour, and whisk to make a roux. Gradually add milk and cream, whisking until thick. Add sharp white cheddar, Gruyere, and mozzarella. Stir until melted.
Combine: Stir in cooked macaroni and mix until coated.
Panko Topping: Mix panko, melted butter, and Parmesan (if using). Spread on top.
Bake: Bake at 350°F (175°C) for 20–25 minutes until golden and bubbly. Optional: Broil for 2 minutes for extra crunch.