Cook pasta according to package directions. Set aside. In a medium pot, melt the butter over medium heat. Add flour, whisking constantly and cook for 1 minute. Slowly add the milk, still whisking constantly. Raise heat to high, bring to a boil and cook for 2-3 minutes until thickened, still whisking (you don't want the bottom to burn!).
Reduce heat to medium-low. Add the cheddar and cream cheese, stirring until completely melted. Add the buffalo sauce and stir. Add the pasta and shredded chicken and stir until everything is coated.