Rub one of the mint leaves inside a high ball glass (or metal cup if you happen to have one handy!) and then add with the remaining 4 mint leaves to the glass along with sugar syrup and a few drops of the bourbon. Muddle. Add a handful of crushed ice to the glass. Place a straw so that it reaches the mint mixture at the bottom of the glass. Pour remaining bourbon over the ice. Add more crushed ice, making a mound that reaches to the top of the glass and leaving about 1 inch of the straw peeking above the rim. Garnish with a mint sprig.