Meanwhile, in 2-quart saucepan, heat butter over medium-high heat until melted. Sprinkle flour on butter; cook and stir constantly 1 minute. Slowly beat in milk with whisk, and continue to stir about 5 minutes or until mixture comes to a boil. Reduce heat to medium; stir in cream cheese, stirring constantly with whisk until smooth. Slowly add Cheddar cheese, 1/2 cup at a time, stirring constantly until cheese is melted. Stir in salt. Serve dip with pretzel knots.