Step 1

Place the blackberries, simple syrup, and lemon juice into a cocktail shaker. Using a muddler (or the back of a wooden spoon), thoroughly mash the ingredients, allowing the blackberry juices to release.

Step 2

Add the rum and three ice cubes to the cocktail shaker. Tightly seal the lid onto the cocktail shaker and shake vigorously for 45-60 seconds.

Step 3

Strain the blackberry cocktail into a double old-fashioned glass. Top with additional ice. Garnish with a lemon slice and straw if you like. Serve immediately.