In the bottom of a cocktail shaker, add the mint leaves and muddle. Fill the cocktail shaker with ice. Add the bourbon, simple syrup, and lemon juice. Fit the cocktail shaker with a tight-fitting lid and shake vigorously.
Fill a glass with ice, and pour the cocktail over the ice. Top off the cocktail with the lemon sparkling water. Garnish glass with a lemon slice and mint sprig, if desired. Enjoy right away while cold.