1. Generously oil the bottom and sides of a 15- x 10-inch jelly-roll pan. Stir together granulated sugar, 1/2 cup corn syrup, and 1/2 cup water in an oiled 3 1/2-qt. heavy saucepan over medium heat until a candy thermometer registers 300° to 310° (hard-crack stage, about 15 to 20 minutes).
2. Remove from heat, and carefully stir in peppermint flavoring until well blended; quickly pour mixture onto prepared jelly-roll pan. Cool 45 minutes or until completely cool. Carefully break into pieces.
3. Carefully brush or dip edges (some edges will be jagged and sharp) of candy pieces with remaining 1/4 cup corn syrup, allowing excess syrup to drip off. Sprinkle with clear sparkling sugar.