In the bottom of a mixing glass, muddle the basil leaves with the gin. (How many leaves you use will depend on how large they are, and how strong you want the basil flavor to be. But feel free to use a lot.)
Add the juice and syrup, then shake hard with ice. Fine strain into an old fashioned glass filled with fresh ice. Garnish with a basil leaf.
Created by Jörg Meyer of Le Lion in Hamburg, Germany.