Add the sugar and water into a small saucepan over medium heat, stirring until the sugar is dissolved. Remove from heat add the rosemary sprigs. Allow mixture to steep, covered, for 20 minutes. Strain out solids and discard. Will keep, tightly sealed and refrigerated, for up to 2 weeks.
Fill a Moscow mule cocktail cup half-way with ice. Combine all the ingredients except the ginger beer. Stir until rum is cooled and well combined. Top with the ginger beer, and garnish with fresh cranberries and a rosemary sprig.
