Make a roux by melting the butter with the flour in a large pot over medium heat. Cook the roux until it resembles wet sand, about 5 minutes Add the milk and whisk constantly until smooth. Add the Cheddar, American cheese and cream cheese, and whisk to a creamy sauce consistency.
Add the sauce to the macaroni and cook to desired consistency. Mix in the kosher salt and some white pepper.
Recipe courtesy of Robert Irvine