Put the blackberries and mint leaves in a cocktail shaker, and muddle using a cocktail muddler or the end of a rolling pin until the mint is bruised and the blackberries have started to release their juice.

Pour in the bourbon and honey, then add a handful of ice cubes. Put the lid on the shaker and shake until the outside feels cold.

Fill a tumbler with crushed ice, then strain in the cocktail. Garnish with a mint sprig and serve.