How to cook
Put the yolks and sugar in a bowl and mix them with a wire whisk. Add 1/4 cup of milk, and cornstarch and stir.
Pour the milk into a saucepan and place on moderate heat to bring to a boil.
Remove from the heat and gradually pour the milk into to the yolk mixture, stirring constantly with a mixer on a low setting.
After all the milk has been added, return the mixture to the hob and boil the cream, stirring constantly with a wire stirrer, until it thickens.
Remove from the heat, add the vanilla extract, lemon zest, and diced butter. Stir until melted.
The finished cream is covered with transparent foil tightly so as not to form a crust.
Before use, beat again with a high-speed mixer until fluffy again.
It can also be served on its own in bowls/cups.