In a medium saucepan, melt butter over medium-high heat.
In a medium bowl, combine egg yolks, lime juice, and sugar, whisking until combined. Add melted butter, whisking constantly.
Return mixture to saucepan. Cook over medium heat, whisking constantly until mixture thickens, 2 to 3 minutes.
Immediately pour mixture through a fine mesh sieve into a small heatproof bowl. Add lime zest, stirring until combined. Place a sheet of plastic wrap on the surface of curd to prevent it from forming a skin while cooling.
Refrigerate until very cold, approximately 4 hours, before using. Store in an airtight container in the refrigerator for up to 2 weeks.
Notes
For a less tart curd, we recommend using regular lime juice instead of Key lime juice.
Recipe by TeaTime Magazine at https://teatimemagazine.com/mango-lime-scones/