5 fresh kaffir lime leaves, chopped, or the zest of one key lime (without pith)
7 tbsp. butter, cut into cubes and chilled
Whisk together sugar and eggs in a 4-qt. saucepan. Add juice, salt, and leaves; stir until smooth. Place saucepan over medium heat; cook, stirring, until mixture thickens to the consistency of loose pudding, about 10 minutes. Whisk in butter, a few cubes at a time, until smooth. Pour through a fine strainer into a large bowl, and chill. If you like, divide between 3 sanitized 8-oz. glass jars with resealable lids and follow the procedure for Canning with a Boiling Water Bath (process filled jars for 10 minutes).