Adjust the oven rack to the lowest position and preheat the oven to CONVECTION/BAKE 375°F. Lightly grease a baking dish with the softened butter.
Make the Bread Crumbs:
In a medium bowl, stir together the melted butter, panko, ¾ cup of the Parmigiano-Reggiano, and the parsley and set aside.
Make the Macaroni and Cheese:
In a large bowl, whisk together the milk, cream, mustard powder, hot pepper sauce (if using), salt, and pepper. Stir in the cheddar and fontina cheeses, then mix in the macaroni.
Pour the macaroni mixture into the baking dish. Cover with a double layer of foil, tightly crimping it around the edges so steam doesn’t escape during cooking. (If steam escapes, the pasta won’t cook through.) Bake for 30 minutes.
Remove the baking dish from the oven, discard the foil, and stir the macaroni and cheese. Sprinkle the bread crumbs evenly over the top and continue to bake until golden brown, about 30 minutes longer. Sprinkle the remaining ¾ cup Parmigiano-Reggiano over the top and set the oven to BROIL/HI. Broil until the top is golden brown, 2 to 4 minutes. Remove the dish from the oven and let stand for 5 minutes. Serve.
Variation: Three-Cheese and Truffle
Add 1 cup of your favorite crumbled blue cheese and as many shaved truffles or as much truffle oil as you like to the milk and cheese mixture.
Variation: Spicy Mac with Bacon Crumbs
Increase the hot pepper sauce to 2 teaspoons and whisk 1 tablespoon (or more to taste) chopped chipotle chiles in adobo sauce into the milk and cheese mixture. For the breadcrumbs, add 2 teaspoons smoked paprika (pimentón) and substitute 1½ cups finely chopped and cooked bacon for ¾ cup of the Parmigiano-Reggiano. (You’ll still sprinkle ¾ cup Parmigiano-Reggiano over the macaroni and cheese before broiling.)