Add all of the ingredients except the ginger beer to a shaker with ice, and shake vigorously for 25 seconds.
Double-strain into a highball glass filled with ice, and top off with the ginger beer.
Garnish with a mint sprig and three blackberries.
*Blackberry mint syrup: Add 1 cup of sugar and 1 cup of water to a small saucepan over medium heat. Stir until the sugar is dissolved. Add 1.5 cups of blackberries, and muddle the berries into the syrup. Add 2 cups of mint leaves and stir. Place in the refrigerator for 24 hours, and strain into a jar. Keeps in the refrigerator for 1 month.