In 1-qt glass measure place butter, flour and salt.
Microwave at High (10) 2 minutes stirring after 1 minute.
Gradually stir in milk.
Microwave at High (10) 3 1/2 to 4 1/2 Minutes, stirring every minute until thick and bubbly.
For thicker sauce, use 3 tablespoons flour instead of 2 tablespoons.