Combine all ingredients in shaker with ice and shake vigorously. Strain into Martini glass and garnish with one large mint leaf.
For mint simple syrup:
In saucepan,bring 1 cup of water to a boil and stir in 1 cup of sugar until it dissolves.Add ¼ cup mint leaves (including stems) to pot, remove from heat, cover, andlet steep for 30 minutes. Strain liquid into separate container, and let cool.