Preheat the oven to 190°C (375°F).

Cut circles from the puff pastry using a pastry cutter. In half of the circles, cut a small hole in the center and place them on the whole circles, pressing lightly.

Place the pastry on a baking sheet lined with parchment paper. Brush with egg yolk mixed with milk and bake for about 20 minutes until golden. Remove from the oven and let cool completely.

Heat the milk in a saucepan until simmering.

In a bowl, mix a small amount of the hot milk with egg yolks, sugar, cornstarch, and salt.

Slowly pour the hot milk into the egg mixture, stirring constantly.

Return the mixture to the saucepan and thicken over medium heat, stirring continuously. Add vanilla extract.

Remove from heat, add butter, and let the cream cool while covered.

Once the pastries have cooled, fill them with the cooled vanilla cream.

Top with fresh blueberries and dust with powdered sugar before serving.