Pour all ingredients except ginger beer into a mixing glass. Shake with ice, strain into a rocks glass over fresh ice and top with ginger beer. Garnish with a mint sprig.
To make the mint syrup: Place 12 fresh mint leaves, 1 cup granulated sugar and 1/2 cup water in a saucepan. Bring to a boil, stirring constantly. Let cool, then sit for a few hours. Strain out mint leaves and store, tightly covered in the refrigerator for up to 2 weeks.