Muddle mint and simple syrup in a rocks glass or silver julep cup. Top with crushed or shaved ice. Pour the bourbon over the top, and stir until the cup frosts. Garnish with a sprig of mint and serve with a straw.
For mint syrup:
Heat the sugar and water in a saucepan over medium heat. Do not boil. Stir until sugar dissolves, about 3 to 5 minutes, then remove the pan from the stove. Add the mint leaves. Gently muddle the leaves against the side of the pan using the back of a wooden spoon. Cover, and allow the mint to steep for 30 minutes to an hour. Strain and pour into a clean jar or bottle. Then, cover and refrigerate.
Recipe adapted from The New Cocktail Hour by André Darlington and Tenaya Darlington, Running Press, a member of the Perseus Books Group.