This recipe was first published in our story on the Pisco Sour.
20ml Lime juice (ideally from aromatic Thai limes)
20ml Simple syrup
20ml Egg white
3ds Angostura bitters
Combine all ingredients except bitters; shake vigorously over ice; strain into a rocks glass*; garnish with bitters.
*Peruvians have a specialised Pisco Sour glass (an undersized old-fashioned glass on a short stem).
Chileans sometimes omit the egg white and bitters. This makes for a simpler but still tasty drink. Most consider egg white essential, though. Aim for a generous centimetre or two of foam; this cloudy nest traps the grape-y aromas and provides a platform for the bitters to work their olfactory magic.
For an extra frothy head, toss the coil from a Hawthorne shaker into the mixing tin and dry-shake. Peruvians just use a blender. Even high-end bars in Peru prepare jugs of Pisco Sour base by whizzing up lime, sugar and egg-white, then fix each drink by adding pisco and shaking over ice.
For maximum authenticity, use Amargo Chuncho bitters from Peru, known for a viscous consistency that sits well atop egg-white foam. Otherwise, Angostura work fine or, for something sweeter and rounder, the cinnamon notes of Fee Brother Whisky Barrel-Aged Bitters make a perfect match.