Add the simple syrup, blackberries, and mint leaves to your julep cup or rocks glass. Press gently 3–4 times.

Add the bourbon to the cup and stir to integrate the flavors with the muddled base.

Fill the cup halfway with crushed ice. Use a long bar spoon (paid link) to stir rapidly for about 15 seconds. Top with more crushed ice, packing it down slightly until it forms a dome top.

Add a fresh mint sprig and a few whole blackberries for garnish, if desired.