In a chilled cocktail shaker add the pisco, lime juice, simple syrup or agave and the egg white. Add a cup or so of ice to the shaker and continue shaking until the mixture is very cold, and foamy, 20 to 30 seconds.
Strain into a rocks or coupe glass, then immediately add a dash of bitters to the top and serve cold.
Notes
You really have a lot of choices, but rocks glasses are the most typical vessel for pisco sours. I like to serve pisco sours in coupe glasses. The drink is typically served straight up, rather than over ice or on the rocks. It gets quite chilled when shaken for a generous amount of time in a cocktail shaker with ice (as well as foamy!).