In a small saucepan over medium heat, melt butter.

Remove pan from heat. Whisk in sugar, zest and lime juice. Add the yolks one at a time, whisking well between additions. 

Return pan to medium-low heat, whisking continuously until mixture thickens and coats the back of a wooden spoon or rubber spatula. 

Immediately press through a mesh strainer into a clean bowl. Cover with plastic wrap, pressed to the surface or transfer to a smaller storage container. Allow curd to cool to room temperature before transferring to the refrigerator. Store chilled until ready to use.