Layer half of potato slices in 1-quart casserole.
Mask with half of sauce.
Repeat with potatoes and sauce.
Dot with butter.
Microwave 25 to 30 minutes on '4', or until potatoes are fork tender.
Let stand 5 minutes before serving.
For ovens without variable heat control, Microwave 23 to 28 minutes on '5', with 2 minute rest periods every 10 minutes.