Preheat a pot on low heat, then add one-fourth cup of butter. Whisk in the flour until well incorporated, forming a roux.
Slowly and gradually stir in one-fourth of the half-and-half, stirring frequently, and bring the mixture to a low simmer. Cook until the mixture thickens, almost 6 to 8 minutes.
Gradually add Velveeta cheese cubes, whisking to melt the cheese. Whisk in three-fourths cup Parmesan cheese, kosher salt and white pepper to gradually mix in the Velveeta. Taste, and to modify the taste, adjust the seasonings if you wish. This makes a some amount of cheese sauce; the cheese sauce can be made up to three days ahead of time and covered and refrigerated.
Cook the macaroni, follow the instructions on the package for the recommended cooking time until they are al dente. Drain and set aside.
While the macaroni is cooking, begin adding the panko topping to the mixing bowl. Mix the panko bread crumbs with the remaining three tablespoons of Parmesan cheese in a small bowl. Drizzle the remaining 2 tablespoons of melted butter over the mixture, stirring well to combine.
Set the oven to convection bake at 450 degrees Fahrenheit.
Warm the cheese sauce over low heat in a large pot and stir in the macaroni until well mixed. Spoon the macaroni and cheese onto a greased 13-by-9-inch baking dish. Sprinkle top of the mac and cheese with the panko topping. Cover the pan with a greased piece of foil.
Now bake the macaroni-and-cheese for 15 minutes to allow the cheese to warm through. then take off on the foil and continue to bake until the topping is a golden brown color, approximately 5 minutes.
