Prepare the Lemon Cream: Heat 1/2 liter of milk with lemon zest until warm, then cool to room temperature.

Whisk eggs, sugar, and vanilla sugar in a bowl until smooth.

Mix in cornstarch until fully combined.

Gradually whisk the cooled milk into the egg mixture and cook over medium-high heat, stirring constantly until thickened. Let it cool.

Prepare the Choux Pastry: Combine water, milk, butter, salt, and sugar in a saucepan. Heat until butter melts.

Add flour all at once and stir over medium heat until a smooth ball forms and pulls away from the pan.

Let the dough cool for 5 minutes. Add eggs one at a time, mixing thoroughly after each addition until the dough reaches a smooth consistency that slowly falls off a spoon.

Bake the Choux Pastry: Preheat the oven to 180°C (356°F).

Pipe or spoon small mounds onto a greased baking sheet. Brush tops with beaten egg.

Bake for 30-35 minutes until golden and puffed. Let cool.

Prepare the Mascarpone Filling: Beat mascarpone in a bowl until smooth and creamy.

Assemble the Cream Puffs: Cut cooled choux pastries in half. Spoon lemon cream and a dollop of mascarpone into each puff.

Serve immediately or refrigerate until ready to enjoy.