Makes one cocktail.
Combine first 4 ingredients in glass over ice. Stir, top with 2 dashes of bitters and a splash of club soda and garnish with rosemary sprig.
Rosemary Simple Syrup: 1 cup water, 1½ cups sugar, 5 sprigs rosemary; Bring water to a boil in a saucepan over moderately high heat. Lower to a simmer and add sugar, stirring until dissolved. Roll rosemary sprigs to release oils before adding to liquid. Simmer for 10 minutes before removing from heat and allowing to cool.