Add blackberries, rhubarb, water, and sugar, to a medium sauce pan over medium heat. Allow the mixture to simmer for 15 minutes, then remove from heat and strain the seeds and fruit pulp out using a fine-mesh sieve.

Place the syrup into the fridge to cool for 1-2 hours.

In a highball glass filled with ice, add vodka and blackberry rhubarb syrup. Fill the remainder of the glass with club soda, then squeeze lime wedge into the drink. Stir to combine.

Using your hands, slap your mint garnish between your palms to “wake up” the mint flavor. Add this garnish to the top of the cocktail and enjoy.