Bring a large pot of water to a boil, and salt it generously. Allow the water to return to a boil and cook the macaroni according to package directions. Drain the pasta.
While the pasta is cooking beat the egg and combine it with the milk in a measuring cup.
Melt the butter in the same pot you used to cook the pasta over medium-high heat. Add the flour and stir frequently for 1 to 2 minutes until the mixture turns golden. Slowly pour in the milk mixture, stirring or whisking constantly. Turn the heat to medium-high, and stirring often, bring the milk to just barely to a simmer. Immediately reduce the heat to low and cook, continuing to stir frequently, until the mixture has thickened slightly, about 3 minutes. Add the Grand Cru, cheddar, Butterkase, and Havarti in any order, but just one handful at a time, stirring after each addition until it is melted. Stir in the mustard and season with the cayenne and salt and pepper as you wish. Add the drained pasta to the cheese sauce and stir until well combined. Adjust the seasonings, and turn into a serving bowl.
This post has been sponsored by Roth cheese. All the opinions expressed are my very own.