Combine the water and sugar in a large non-reactive saucepan and stir to dissolve the sugar, add mint and bring to a boil. Remove from heat, cover and let sit until cool. Strain the mint syrup through cheesecloth (a T-shirt works too) and place in an empty sterilized whiskey bottle. Store in the refrigerator until ready to serve.
To serve, pack a chilled Mason jar (put them in the freezer before the party) with crushed ice, add bourbon to taste (usually 2 jiggers) and fill the jar with mint syrup. Rub a few leaves of mint around the rim of the jar and garnish with a sprig of fresh mint.
Makes 1 quart