In a cocktail shaker, add the 3 blackberries and the simple syrup, then gently muddle together. Add the bourbon, lemon juice, and bitters, then fill with ice. Cover and shake vigorously until chilled.
Fill a highball glass with ice, then double strain the cocktail into the glass. Top with the ginger beer. Garnish with a blackberry and mint sprig. Serve immediately.
Fill a large