To a medium-sized saucepan add the lemon zest, lemon juice, sugar, egg & yolks, and salt.  Whisk well to combine.

Set the pan over low heat and cook, while stirring continuously, until the mixture thickens and reaches 165-170F on a candy thermometer or coats the back of a spoon and leaves a line when you run a finger down it. This will take about 8-10 minutes.  

When it's reached the proper temperature/ consistency strain through a fine-mesh sieve directly into a heat-proof bowl, helping the mixture to completely flow through with your spoon if needed. 

Whisk in the cubed butter until completely combined then cover the curd with a layer of plastic wrap directly touching the surface, so it doesn't create a skin on top.  Cool to room temperature then store in a small jar/ container with a tight lid in the refrigerator.  It will keep well refrigerated for up to two weeks if stored airtight.