1: Boil water in a medium sized pot.
2: Combine sugar, eggs, and zest with a whisk in a medium bowl.
3: Gradually add citrus juices until combined.
4: Place bowl over boiling pot of water, whisk occasionally.
5: Cook until you whisk lines form in the curd.
6: Stir in butter.
7: Strain into container.
8: Chill until ready to use.
