Line a 10x15 inch cookie sheet with aluminum foil.
Cook the sweet potatoes in the microwave until done, about 4 minutes per potato. When the potatoes are cool, peel, cut in half lengthwise and slice about 1/4 inch.
Place the potato slices in a single layer on prepared cookie sheet. Slice the butter into about 1/8 inch pats and place all over the sweet potatoes. Sprinkle the sugar on the potatoes and butter. Place under broiler until golden and bubbly, about 5 to 8 minutes.