In 4-cup glass measure, place butter.
Microwave at high (10) 1/2 to 1 minute until melted.
Blend in flour and salt.
Stir in milk until smooth.
Microwave at high (10) 4 to 6 minutes, stirring every minute until thickened.
Add cheese; stir until completely melted.
In 2-quart casserole, combine cheese sauce and macaroni, mixing well.
Microwave at medium high (7) 5 to 10 minutes, stirring every 3 minutes.