In a medium saucepan stir together sugar and water. Heat and stir over medium heat until sugar dissolves. Remove from heat. Stir in 1 cup mint. Let stand for 1 hour; remove and discard mint. Transfer syrup to a small bowl; cover and chill several hours or overnight.
To serve, in a pitcher stir together the syrup and bourbon. Stir in sparkling water. Serve over ice with fresh mint sprigs.