Preheat the oven to 375°. Bring a large pot of salted water to a boil. Add the pasta and cook until al dente. Drain and return the pasta to the pot.
Meanwhile, in a large saucepan, melt 4 1/2 tablespoons of the butter. Whisk in the flour and cook over moderate heat, whisking constantly, until smooth, about 1 minute. Add the milk and whisk over high heat until the mixture comes to a boil. Reduce the heat to moderate and simmer, whisking, until thickened, about 2 minutes.
Remove the saucepan from the heat and stir in the cheeses until smooth. Season the cheese sauce with cayenne, salt and black pepper. Fold the pasta into the cheese sauce and scrape the mixture into a large glass or ceramic baking dish.
In a medium heatproof bowl, melt the remaining 2 tablespoons of butter in the microwave. Stir in the panko and season with salt and pepper. Sprinkle the panko over pasta. Bake for about 40 minutes, until the top is golden and the sauce is bubbly.
Preheat the broiler. Broil the mac and cheese 4 inches from the heat for about 2 minutes, until the top is golden brown. Remove from the oven and let stand for 10 minutes before serving.