Boil water and cook pasta according to package directions.
Meanwhile, melt butter on medium heat in a large saucepan. Add flour, mix in to butter and cook for 2 minutes. Slowly add milk a little at a time, whisking continuously. Add about one third of cheese at a time, whisking until thoroughly mixed each time.
Fold in pasta and serve.
Store leftovers in an airtight container in the fridge for up to 5 days.