As seen in Le Lion in Hamburg.
1 bunch basil (1-2 handfuls)
2 cl simple syrup (Sugar / Water dissolved over heat)
8 cl gin (Hendrick’s or similar quality gin)
Stuff the basil into a cocktail shaker, followed by the lemon. Muddle the lemon and the basil, smashing the ingredients and squeezing the lemon.
Add simple syrup, mix.
Fill shaker with ice, top with gin. Shake vigorously until very cold — about 30 or so seconds. A good rule of thumb is to stop when the shaker begins to frost over and it’s almost too painfully cold to hold. Double-strain into a rocks glass filled with ice, squeezing every bit if liquid through a fine mesh strainer with a bar spoon.
Give the glass a stir and add more ice if necessary. Garnish with another basil leaf.