Lightly beat eggs in a mixing bowl.
Add the cornstarch and lemon zest. Whisk to combine and set aside.
Heat milk with sugar and Vanilla sugar in a saucepan until steaming hot. Then take it off the heat.
Pour the lemon juice into the bowl containing the eggs. Do it slowly while constantly whisking the eggs. It will thicken slightly.
Pour the hot milk into the eggs very slowly. Again, whisking the eggs constantly. Icorporate at least ⅔ of the milk with the eggs. The mixture will look slightly curdled, no worries, it will become smooth in the next steps.
Transfer the mixture back to the cooking pot.
Cook the cream until it thickens over medium-low heat, stirring often with a whisk. Then remove from the heat.
Stand the cream for 10-15 minutes to cool slightly. Stir now and then so it won't form a skin on top.
Stir in the butter.
Transfer the cream to small bowls or glasses and allow to cool. See Notes below.
Cool. Let the cream reach room temperature, then refrigerate for 4-5 hours before serving.
Serve with vanilla or cinnamon biscuits, fresh strawberries, or other red fruit, or keep it simple and serve as is!