Fill large martini glass with crushed ice and set aside. Reserve one mint sprig for garnish. Remove and discard stems from remaining mint and place leaves in a small bowl. Add sugar, and, using back of a spoon, crush mint; stir, adding a few drops of water so the sugar fully dissolves. Add small amount of ice, then the liquors. Stir to combine, and strain mixture into glass. Garnish with mint sprig.