Prepare the Milk Mixture: In a saucepan, combine the milk, lemon zest, and vanilla sugar. Cook over medium heat until it begins to boil.

Prepare the Egg Mixture: In a separate bowl, whisk together the egg yolks, sugar, and cornstarch until smooth.

Combine and Cook: Gradually pour the hot milk mixture into the egg mixture, whisking constantly to avoid curdling.

Return the mixture to the saucepan and cook over medium heat, stirring constantly, until it thickens.

Cool the Custard: Remove from heat and let the custard cool to room temperature.

Mix the Final Ingredients: In a mixing bowl, combine the cooled custard, lemon juice, and condensed milk. Beat until the mixture is thick and smooth.

Serve: Pour the custard into serving dishes and decorate with cinnamon, cocoa, or chocolate.

Place in the refrigerator for 30 minutes before serving.