Remove the zest from the lemon. Place it in a saucepan with 500 ml milk and bring to a boil over low heat. In the meantime, whisk the eggs, 120 g sugar and vanilla sugar in a bowl. Whisk until the sugar has dissolved.
Add the starch and mix. Remove the zest from the milk and pour into the egg mixture. Cook over low heat until it thickens.
Put it in a bowl and let it cool. In the meantime, mix the milk and water in a saucepan. Add the butter, sugar and a pinch of salt. Cook until the butter has dissolved.
Then stir in the flour and make a dough. Cook for about 2 minutes, stirring constantly. Place in a container, let cool and mix in the remaining eggs.
Mix and place in a baking bag. Line with baking paper. Pour the dough onto a baking sheet and shape the cookies into squares measuring 5×5 cm.
Bake at 180 degrees for about 35 minutes. Cut out the cookies, fill with mascarpone and assemble. Sprinkle with icing sugar.
