Place the mint leaves in the bottom of an old-fashioned glass and top with the sugar. Add the mint leaves and gently bruise with a wooden muddler or a wooden spoon. Take care not to overwork the mint, but swab the sides of the glass with the mint's aromatic oils. Add the bourbon and pack tightly with crushed ice. Fill the glass with crushed ice and stir until the outside of the glass frosts.
Top with more crushed ice to form an ice dome and garnish with a few drops of bitters, if desired, and a mint sprig.
(Courtesy: Chef Lee Tuck Sung, Royal China)