In a medium bowl, whisk the egg yolks and eggs until evenly blended and set aside.  In a medium saucepan, heat the sugar, lemon zest, lemon juice and salt until the sugar has melted.

Slowly whisk in the the hot liquid into the eggs to temper them.  Return the egg mixture back into the saucepan and whisk on very low heat until the curd thickens.  Be patient as it can take up to an hour.

It is possible to cook it faster, but you will run the risk of scrambling the eggs.  When the curd has thickened (it should coat the back of a spoon) remove the saucepan from the heat and whisk in the butter.

Spoon into a clean bowl and set the bowl in an ice bath, stirring until the curd has cooled.  Then cover the top with food film, pressing it down directly on the curd.  Place in the refrigerator to set up completely.