Place mint leaves and Mint Syrup* in a chilled julep cup. Gently press leaves against cup with back of spoon to release flavors. Pack cup tightly with crushed ice; pour bourbon over ice. Insert straw, place mint sprig directly next to straw, and serve immediately. Sprinkle with powdered sugar, if desired.
*To make Mint Syrup, bring 1 cup sugar and 1 cup water to a boil in a medium saucepan. Boil, stirring often, 5 minutes or until sugar dissolves. Remove from heat; add 10 to 12 fresh mint sprigs, and let cool completely. Pour into a glass jar; cover and chill 24 hours. Remove and discard mint.