In a cocktail shaker or sturdy glass, muddle the blackberries until completely juicy and broken down.
Add the bourbon and simple syrup.
Add the fresh mint leaves and press them gently once or twice. Do not over-muddle the mint or it will turn bitter.
Add ice and shake well until thoroughly chilled.
Fill a glass with crushed ice. Strain or pour the cocktail over the ice.
Garnish with a fresh sprig of mint and a few blackberries. Serve immediately.