Wash sweet potatoes. Cut thin slices in each potato, but do not cut all the way through. Leave about 1/4 inch of uncut potato at the bottom to hold it together.
Rub each potato with the butter, trying to push as much of it as possible into the cracks. Set buttered potatoes into a casserole dish.
Bake in preheated oven. Every 15 minutes or so, use a baster to suck up any melted butter and squirt it right back on top of the potatoes. Cook until the potatoes are very tender, 45 minutes to an hour. During the last 10 minutes of cooking time, sprinkle the sugar and salt onto the potatoes.