Preheat the oven to 400°F. Place a rack in the bottom third of the oven, and butter a 3-quart baking dish. Set a large pot of unsalted water over high heat.
While the water heats, melt the butter in a large saucepan. When the butter has finished foaming, stir in the flour, whisking until the flour takes on a little color, about 3 minutes. Take the pan off the heat and pour in the milk slowly, while whisking continuously. Return the pan to medium heat. Stir until the mixture begins to thicken (about 5 minutes) and then take the pan off the heat again; stir in the cream, mustard, and three-fourths of the cheese. Stir in the salt, a few grinds of pepper, and the hot sauce. Set the sauce aside.
Cook the pasta just until al dente. Drain (don’t rinse) and quickly stir the pasta into the cheese sauce, then pour into the prepared pan, scraping all the cheese sauce into the dish. Sprinkle the remaining grated cheese over the pasta. Sprinkle the panko over the cheese, and sprinkle the Parmesan on top of that.
Bake until the mixture is bubbling on the edges and showing some golden brown color on top, 25 to 35 minutes. Let the dish cool for at least 10 minutes before serving.