In a large shaker, muddle together the 6 blackberries and 6 mint leaves. Add the vodka, cassis and lime juice. Add several cubes of ice to the shaker and with the lid securely fitted, shake vigorously for at least 20 seconds. Strain the liquid into two low ball glasses or copper mule mugs. Add a few cubes of ice to each glass, top with ginger beer and garnish with a blackberry and sprig of mint. Enjoy!
Although the Moscow mule traditionally is served in a copper mug, I recommend serving this variation in a clear glass to show off the gorgeous color of the muddled blackberry.