Add the sugar and zest to a medium bowl and using your finger tips mix together the sugar and zest.
Add the lemon juice and place the bowl over a pot of simmering water. Heat the mixture until the sugar is dissolved. Occasionally, stir with a rubber spatula scrapping the bottom of the bowl.
Add the butter and heat until the butter is melted.
In another bowl gently whisk the egg yolks.
Gradually whisk the warmed sugar/lemon juice mixture into the egg yolks and return the mixture to the first bowl.
Place the bowl back over the simmering water and stir until the mixture begins to thicken.
Strain through a mesh strainer into a bowl. Press plastic wrap on the surface of the curd and refrigerate until ready to use. Or freeze for longer storage.