To a small, non-reactive saucepan, add the lemon juice, lemon zest and cornstarch and mix. Add butter and sugar and bring the mixture to a boil over medium heat. Boil and stir for one minute. Reduce heat to low.

In a small bowl, whisk the egg yolks until smooth. Add a small amount of the hot lemon mix to the bowl and stir until fully incorporated.

Add egg mixture back to saucepan and cook over low heat, stirring constantly until thick, about 5 minutes.

Place sauce in a small bowl and put plastic wrap directly on the surface of the sauce to prevent a skin from forming as it cools. Allow it to cool to room temperature and then refrigerate for 3 hours.