Lime Leaf Sour Recipe

Combine sugar and water in a small saucepan over medium heat. Stir until the sugar fully dissolves and the mixture comes to a gentle simmer.

Add the bruised kaffir (lime) leaves to the saucepan, reduce heat to low, and simmer very gently for 5 minutes to extract the essential oils and aroma. Do not boil vigorously — you want a gentle infusion.

Remove from heat and let the leaves steep in the syrup as it cools to room temperature, about 20–30 minutes, for a stronger flavor.

Strain the syrup through a fine mesh strainer to remove the leaves. Transfer to a clean bottle or jar. Refrigerate; syrup will keep 2–3 weeks.

Prepare (if using egg white)

If you want the classic sour foam, separate the egg white and set aside. If avoiding raw egg, skip this step or use 0.5 oz pasteurized liquid egg white.

Add the white rum, fresh lime juice, lime leaf-infused simple syrup, and egg white (if using) into a cocktail shaker without ice for a dry shake. Dry shake vigorously for 12–15 seconds to emulsify and create a foam.

Open the shaker, add a generous handful of ice (about 1 cup), then shake again vigorously for 10–15 seconds to chill and dilute the cocktail.

Double-strain the cocktail into a chilled coupe or rocks glass (use a fine mesh strainer on top of the Hawthorne strainer to remove ice shards and any leaf bits).

Carefully add 2 dashes of Angostura bitters atop the foam. Using a toothpick or bamboo skewer, gently swirl the bitters to create a simple decorative pattern if desired.

Garnish with a fresh lime leaf laid on the foam or a small lime wheel perched on the rim and a light sprinkle of grated lime zest if you like extra aroma.