How to Make It
With a vegetable peeler, pare green part of peel from 4 limes. Put peel and sugar in a food processor and whirl until peel is very finely chopped; let stand, covered, 30 minutes or up to 6 hours. Ream juice from limes. Measure 1 cup; save remainder for other uses.
Add the 1 cup lime juice to sugar mixture in food processor; whirl until sugar dissolves. Pour mixture through a fine strainer into a 2- to 3-quart pan, pressing liquid from peel; discard peel. Add eggs and whisk to blend; add butter.
Stir over medium-low heat until mixture thickly coats a metal spoon, about 15 minutes. Scrape into a bowl, cover, and chill until cold, at least 1 hour, or up to 3 days. Stir before using.
Nutritional analysis per tablespoon.