Special equipment: 4 chilled martini glasses and 4 cocktail picks
Muddle the blackberries and basil in a pitcher.
Add the club soda, vodka, cassis, lime juice and agave and stir to combine. Chill for at least 1 hour and until ready to serve.Thread a basil leaf and blackberry onto each cocktail pick.
Serve the spritzers over ice and garnish with the cocktail picks.