Grilled Glazed Sweet Potatoes

Scrub sweet potatoes well. Place in a large pot and cover with cold water. Bring water to a boil, reduce heat, and simmer until almost ternder. about 40 minutes (they should be a little undercooked). Remove from heat and drain; peel and set aside. (Sweet potatoes can be prepared 1 day in advance up to this point) Combine balsamic vinegar and sugar in a small saucepan over high heat. Simmer mixture until it is thick and syrupy, or reduce by one-half, about 10 minutes. Remove form heat, add butter and cool. If using grill, heat grill to high. Cut sweet potaotes crosswise into 1/2 inch thick slices. Brush tops of slices with oil and place, brushed side down, on grill until marked, about 2 minutes. While slices are cooking. brush the other sides with remaining oil and turn over. Grill until marked, about 2 minutes more. Brush grilled sweet potatoes on both sides with vinegar glaze and serve. If using broiler, heat broiler to high. Arrange sweet potatoe slices on a baking sheet and brush tops with glaze; no oil is necessary. Place baking sheet under broiler and broil until galze starts to bubble, 3 to 4 minutes. Remove from oven, turn potatoes over, and brush with glaze. Broil for 3 to 4 minutes more.