Preheat oven 375.
Melt butter in a 10 inch cast iron skillet over medium-low.
Whisk in flour and cook for two minutes.
Vey gradually add milk and raise heat to bring mixture to a boil. Stir constantly.
Add salt.
Once it boils, lower to a simmer for 5 minutes to thicken it.
Whisk in cheese and once smooth add the pasta.