In a mixing bowl, combine butter, sugar, and flour until a soft dough forms. Roll the dough between two sheets of parchment paper to about 0.5 cm thick.

Freeze the dough and then cut out circles that will fit the tops of your pastries. Keep them in the freezer until ready to use.

In a saucepan, heat together water, milk, butter, salt, and sugar until it comes to a boil. Remove it from heat and quickly add all the flour.

Stir vigorously for about 2-3 minutes until a smooth ball forms and a film appears at the bottom of the pan. Allow it to cool for 5-10 minutes, then add the eggs one at a time, mixing well after each addition.

The finished dough should slowly flow off the spoon in a triangle shape. Transfer the dough into a pastry bag and refrigerate for at least 2 hours.

Pipe the choux pastry onto a baking sheet lined with parchment paper, leaving some space for them to expand; aim for about 4-5 cm in diameter.

Top each pastry with a circle of cracquelin and bake in a preheated oven at 170°C (340°F) for 45-55 minutes until they are golden brown.

Do not open the oven door for the first 40 minutes! Let them cool on a wire rack.

In a bowl, whisk the egg yolks with sugar, vanilla sugar, and cornstarch. Heat milk in a saucepan and gradually incorporate it into the egg mixture.

Return this mixture to the saucepan and cook over low heat, stirring constantly until it thickens. Remove from heat and add in butter, allowing it to cool to room temperature.

Refrigerate for 2-3 hours. Once chilled, whip with the mascarpone until light and airy.