Add blackberries to the cocktail shaker and muddle until they release their juices.
Pour in the gin, lemon juice, and simple syrup.
Fill the shaker with ice and shake for 15-20 seconds.
Strain the mixture into a Collins glass filled with fresh ice.
Top with club soda and stir gently with a bar spoon.
Garnish with fresh blackberries and a lemon wheel.
Notes
The drink works best with fresh blackberries during peak season (July-September). Frozen blackberries can work in a pinch.Simple syrup can be made at home by mixing equal parts sugar and hot water until dissolved.Store extra simple syrup in an airtight container in the fridge for up to 2 weeks.For a non-alcoholic version, replace gin with tonic water or additional club soda.