Place the mint leaves, lemon, and simple syrup in the bottom of a cocktail shaker or mason jar. With a muddler or the handle of a wooden spoon, mash 4–5 times, just enough to release juices and oils.
Add the bourbon and top with a small handful of crushed ice. Cover and shake vigorously for 30 seconds to combine. Strain into an old fashioned or similar glass. Pile more crushed ice on top. Garnish with additional mint sprigs. Enjoy immediately.
Notes
For a St. Patrick's Day spin, use Irish whiskey instead of bourbon and replace the lemon with 1/4 of a lime.