In a medium saucepan, add the milk and lemon zest. Warm gently over low heat until just hot (do not boil), then remove from heat and let sit for 10 minutes to infuse.

In a mixing bowl, whisk together the eggs, granulated sugar, vanilla sugar, and cornstarch until smooth and lump-free.

Gradually pour the warm milk into the egg mixture while whisking constantly to avoid curdling.

Return the mixture to the saucepan and cook over medium-low heat, stirring continuously until it thickens into a silky, custard-like cream (about 5–7 minutes).

Remove from heat. Optional: strain to remove zest for an ultra-smooth finish.

Pour into individual cups or ramekins and let cool slightly before refrigerating. Chill for at least 1 hour before serving.