This syrup can be used to make soda or as its own for cocktails.

1/4 cup lemon zest (from 6 lemons)
1/4 cup lime zest (from 8 limes)
20 kaffir lime leaves
2 stalks lemongrass, roughly chopped
3/4 cup Simple Syrup (click for recipe)
6 cups soda water, divided

Combine citrus zest, lime leaves, lemongrass, and 2 cups water in a small saucepan. Bring to a boil, reduce heat, and simmer for 5 minutes. Strain lemongrass mixture into a jar and chill. Fill a 12-oz. glass with ice cubes. Add 2 Tbsp. lemongrass mixture and 1 Tbsp. simple syrup. Top with soda water (about 1/2 cup) and stir to combine. Repeat to make 11 more sodas.

Via Bon Appetit, http://www.bonappetit.com/recipes/2011/05/lemongrass-lime-leaf-soda :

"Chicago chef Graham Elliot uses aromatic kaffir lime leaves to make this sparkler from his sandwich shop, Grahamwich. They're sold frozen, and sometimes fresh, at Asian markets. If unavailable, substitute an additional ½ tsp. lime zest for each lime leaf. Lemongrass can be found in the produce section of better supermarkets and at Asian markets."