Cook sweet potatoes in boiling water for 45 minutes, or until tender. Let cool until they can be handled, then peel and cut into 1/2-inch slices. Layer slices in a lightly greased 12x8x2-inch baking dish. Combine butter and sugars; spoon over sweet potatoes. Cover tightly and bake for 30 minutes, or until thoroughly heated. Sweet potato recipe serves 10 to 12.