Place the blackberries, mint leaves and simple syrup in a cocktail shaker and muddle with a cocktail muddler.

Add the lime juice and the rum.

Fill the shaker with ice.

Shake and strain into a glass filled with ice.

Top with the club soda.

Garnish with extra blackberry and mint leaves.

For the simple syrup, in a small saucepan, boil sugar and water over medium high heat until sugar dissolves.

Remove from heat and let cool.