Step 1In a medium, heavy-bottomed saucepan, melt one-fourth cup butter over low heat. Whisk in the flour to form a roux.
Step 2Slowly whisk in the half-and-half and bring the mixture to a low simmer, whisking frequently. Cook until the roux thickens, 6 to 8 minutes.
Step 3Slowly add the Velveeta cheese cubes, whisking to melt the cheese. Whisk in three-fourths cup Parmesan cheese, kosher salt and white pepper. Taste, and adjust the seasonings if desired. This makes a scant 4 cups cheese sauce; the cheese sauce can be made up to 3 days ahead of time and covered and refrigerated before using.
Step 4Cook the macaroni according to the instructions on the package until al dente. Drain and set aside.
Step 5While the macaroni is cooking, make the panko topping: In a small mixing bowl, mix together the panko bread crumbs and remaining 3 tablespoons Parmesan. Drizzle over the remaining 2 tablespoons melted butter, stirring well to combine.
Step 6Heat the oven to 450 degrees.
Step 7In a large pot, heat the cheese sauce over low heat to warm, then gently stir in the macaroni until well mixed. Spoon the macaroni and cheese into a greased 13-by-9-inch baking dish, then sprinkle over the panko topping. Cover the pan with a layer of greased foil.
Step 8Bake the macaroni and cheese for 15 minutes to warm through, then remove the foil and continue to bake until the topping is golden brown, about 5 minutes.