Place mint leaves and syrup in a chilled julep cup, and muddle. Pack the cup tightly with crushed ice, add the bourbon, top with soda water and garnish with a mint sprig.
How To Make Mint Syrup:
Place 1-1/2 cups sugar and 1-1/2 cups water in a small saucepan. Bring the mixture to a boil and stir until the sugar is completely dissolved, about 2 minutes. Remove from the heat, add about 15 fresh mint sprigs, and allow to cool completely. Cover and refrigerate for 24 hours, then strain the syrup and discard the solids.