Strain egg yolks into a pot and beat lightly with a wire whisk. Whisk in the sugar and give it a good mix until lighter in color. Stir in lemon juice gradually and add the zest
Heat on low, while stirring constantly with a wire whisk until mixture thickens, just starts to bubble and coats back of wooden spoon.
Remove pot from heat, then add the cold, cubed butter and mix until melted.
This is optional but I like to strain the final mixture to remove any bits of zest, etc.
