1. Wash sweet potatoes. Prick each 2 or 3 times with fork to allow steam to escape. Arrange in oven at least 1 inch apart. Microwave at HIGH (100%) until fork-tender, 8 to 10 mins. Cover and let stand 5 minutes. 2. Peel and slice potatoes. Place in 2-quart casserole. Add 1/4 cup butter and the salt.