Make pasta according to package, drain and set aside.
In a medium size sauce pan, add the milk and cook until simmering, making sure to stir constantly so it doesn't burn on the bottom. Whisk in flour one tablespoon at a time. Whisk until no lumps remain and milk begins to thicken.
Stir in salt and ground mustard. Add in cheese, one cup at a time. Make sure cheese is melted and incorporated before adding the remaining cups of cheese. Stir in the butter.
Stir to incorporate all ingredients until smooth and let cheese mixture bubble a little.
Pour cheese mixture over pasta and stir to coat. Add any additional salt if needed.
Serve and enjoy!