Angostura bitters – 2–3 dashes on foam (for garnish)
Instructions
Make a quick simple syrup: Combine the sugar and water in a small saucepan over low heat, stirring just until the sugar fully dissolves, 1–2 minutes. Remove from heat and let cool briefly.
Juice the limes and strain to obtain 1 ounce (2 tbsp) of clear juice; discard pulp and seeds.
Add the pisco, the strained lime juice, 1.5 tbsp of the simple syrup from Step 1, and the white from 1 large egg to a cocktail shaker. Seal and dry-shake (no ice) vigorously until slightly foamy, 10–15 seconds.
Add the ice cubes to the shaker. Seal and shake hard until the shaker is very cold and the mixture is thick and frothy, 12–15 seconds.
Double-strain into a chilled coupe or small rocks glass.
Dot the foam with 2–3 dashes of Angostura bitters. Serve immediately.