This classic comfort food is perfect on a cold winter day.
Preheat oven to 350 degrees.
Boil a large pot filled partway with salted water, then cook the pasta to the package's instructions for al dente, erring on the side of undercooked.
In a medium saucepan, melt the butter over low heat. Whisk in the flour and keep it moving for about five minutes, ensuring that it is free of lumps. Stir in the mustard and paprika and whisk for 30 seconds more. Add the milk and raise heat to medium-low. Stir regularly for 5 minutes.
Temper the egg by adding 1/3 cup of the hot mixture to the beaten egg, while stirring rapidly with a fork. Once they have incorporated, add another 1/3 cup while stirring. Pour the tempered egg into the saucepan, then stir in 3/4 of the cheese. Season with salt and pepper. Fold the macaroni into the mix and pour into a 2-quart casserole dish. Top with remaining cheese and panko crumbs if desired.
Bake for 30 minutes.