Pour all the ingredients, except the ice, in the cocktail shaker and shake vigorously for about 30 seconds. Add ice and then shake for another 30 seconds. Strain into a glass and garnish with a sprig of frosted rosemary. To make frosted rosemary: wet a few sprigs and roll them in sugar (superfine works best, but regular granulated will do). Store in an airtight container.
In a saucepan, combine water and sugar, heat over medium heat until sugar is dissolved. Turn off heat and drop rosemary sprigs in the syrup, cover and allow to steep for 30 minutes. Remove rosemary and strain mixture into a jar. Store in the fridge for up to 1 week.