Ingredients:

  Amount  Measure       Ingredient -- Preparation Method
--------  ------------  --------------------------------
   8                    sweet potatoes
   1      cup           balsamic vinegar
     1/4  cup           sugar
   1      tablespoon    butter -- cold
   2      tablespoons   olive oil

Preparation:

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 tender, 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 reduced by one-half, about 10 minutes. Remove from heat, add butter and cool. If using grill, heat grill to high. Cut sweet potatoes 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 more minutes. Brush grilled sweet potatoes on both sides with vinegar glaze and serve. If using broiler, heat broiler to high. Arrange sweet potato slices on a baking sheet and brush tops with glaze; no oil is necessary. Place baking sheet under broiler and broil until glaze starts to bubble, 3 to 4 minutes. Remove from oven, turn potatoes over, and brush with glaze. Broil for 3 to 4 more minutes.