Special equipment: a cocktail shaker, muddler, 3 Mason jars, toothpicks
Add the lime juice, mint leaves, ginger and 9 of the blackberries to a cocktail shaker. Muddle until the mint breaks down into small flecks, the blackberries into small pieces and the ginger into pieces half their original size.
Add the vodka and some ice to the shaker; shake until combined. Strain into three Mason jars. Pour 4 ounces of the ginger beer over each cocktail. Garnish with a mint sprig and 2 blackberries held together with a toothpick.