Place peaches cut-side down on grill and cook 3 to 4 minutes. Brush bottoms lightly with remaining butter and turn peaches over. Sprinkle sugar mixture over cut side of peaches, cover grill (or use cooking/steamer lids) and cook until sugar has melted and caramelized and peaches are easily pierced with a sharp paring knife, 3 to 6 minutes longer.
