Cook the macaroni until it is very firm, almost too firm to eat right out of the pot. Drain and set aside.

Beat the egg in a small bowl.

In a large pot, melt the butter over medium-low heat and sprinkle in the flour. Whisk constantly and cook the mixture for 5 minutes, making sure it does not burn.

Slowly pour in the milk and add the dry mustard while whisking until the mixture is smooth. Cook for 5 minutes until the sauce becomes very thick. Reduce the heat to low.

Take 1/4 cup of the hot sauce and slowly pour it into the beaten egg, whisking constantly to prevent the egg from cooking. Whisk the mixture until smooth.

Pour the egg mixture back into the pot with the sauce, whisking constantly until fully incorporated and smooth.

Stir in the grated cheese until melted and smooth. Add kosher salt, seasoned salt, black pepper, and any optional cayenne, paprika, or thyme. Taste and adjust salt and seasonings as needed.

Add the drained macaroni to the cheese sauce and stir well to coat evenly.

Pour the mixture into a buttered baking dish, top with extra cheese if desired, and bake at 350°F (175°C) for 20 to 25 minutes until bubbly and golden on top.

Remove from the oven and serve hot.