- In a mixing glass, muddle the blackberries and mint leaves together with the sugar to release their juices.
- Add the lime juice and white rum to the mixture.
- Fill a cocktail shaker with ice, pour in the muddled mixture, and shake well.
- Strain the mixture into a highball glass filled with ice.
- Top with club soda and stir gently to combine.
- Garnish with a sprig of mint and a few blackberries.