Prepare the day before. Preheat the oven to 300°F (150°C).

In a saucepan, combine the water and milk, then add the butter, salt, and sugar.

Heat over low heat for 5 minutes.

Once the butter is fully melted, bring to a boil, then remove from heat and add the sifted flour all at once.

Mix until a dough forms.

Dry the dough over high heat in the saucepan for 1 minute, stirring constantly.

Transfer the dough to a stand mixer fitted with the paddle attachment. Mix on medium speed until no more steam escapes.

The goal is to remove as much moisture as possible so you can add more eggs, which helps the pastry puff up during baking.

Beat the eggs as you would for an omelet, then gradually pour them in a thin stream into the dough while mixing on medium speed.

Transfer the dough to a piping bag and pipe rings onto a silicone baking mat.

For the pastry cream: In a saucepan, bring the milk and cream to a boil.

In a bowl, mix the egg yolks and sugar, then add the cornstarch.

Pour some of the hot liquid over the egg mixture, then return everything to the saucepan and cook until thickened.

Remove from heat and add the frozen butter and softened gelatin, then the chestnut cream. Blend with an immersion blender, cover with plastic wrap directly on the surface, and refrigerate overnight.

Assembly day: Cut the tops off the choux rings, then whip the pastry cream in a stand mixer for 1 minute.

Transfer the cream to a piping bag fitted with a star tip. Add a few blueberries to the bottom, pipe in the chestnut cream, replace the tops, and dust with powdered sugar.

Refrigerate until ready to serve.