Stir together egg yolks and sugar in a medium sized heavy saucepan. Pour in lime juice.
Stirring constantly, cook over medium heat until mixture thickens enough to coat the back of a spoon. Do not allow mixture to boil. It will curdle your eggs
After the mixture thickens, remove from heat. Stir in butter and vanilla.
Send mixture through a fine mesh strainer and stir in lime zest.
The lime curd will be yellow. This is totally fine, but if it weirds your sister out to have yellow lime curd like it does mine, then you can add a drop or two of green food coloring.
Cover with plastic wrap right down on the curd to prevent a skin. Refrigerate for at least 1 hour.