Combine lemon juice, granulated sugar, lemon zest, and unsalted butter in a heavy-duty saucepan.  Over medium heat, bring lemon juice mixture to a boil. Beat whole eggs in a medium size bowl. 

Slowly stream the boiling lemon juice mixture into the whole eggs, whisking constantly.  Pour egg mixture back into the sauce pan.  Cook over medium-low heat, stirring constantly until the mixture thickens.   Be careful to not let the mixture boil, as it will curdle. The mixture should coat the back of a spoon.  Your finger should leave a clear path when passed across the spoon.

Strain the lemon curd through a sieve into a bowl to cool.  Place a piece of plastic wrap directly on the curd’s surface.  Refrigerate until cold.  Use as needed.