In a cocktail shaker, combine the pisco, freshly squeezed lime juice, simple syrup, and egg white (if using).
Dry shake (shake without ice) for about 15 seconds to create froth from the egg white.
Add ice to the shaker and shake vigorously for another 15-20 seconds until well chilled.
Strain the cocktail into a chilled glass.
Garnish with 2-3 dashes of Angostura bitters on top of the froth.
Decorate with a lime slice or cherry if desired.
Enjoy your meal!