Combine basil leaves, simple syrup and lemon juice in a cocktail shaker.Crush the basil till bruised with a muddler, which will release the oils. The juice might start to turn green, this is ok!
Pour in the gin and ice cubes, place lid on top of the shaker.
Shake the container until the outside of the cocktail shaker is freezing cold.This is how you know the drink is cold enough!
Strain into a tall cocktail glass, add additional ice to the top and enjoy!