In the bottom of a julep cup, muddle mint leaves and simple syrup. This will extract the oil from the mint to flavor the cocktail. Add in lime juice and stir.
Liberally fill glass with crushed ice.
Pour gin over ice. Lightly stir.
Garnish with mint + enjoy!
JC's note: using a good quality gin (I like Bombay Sapphire) and fresh lime juice makes all of the difference in this cocktail. Also, if you are in Kentucky (Louisville specifically), be sure to grab your mint a few days early -- it can be hard to find in stores around here right before Derby!