Preheat your oven to 190 degrees C (374 degrees F) and line a baking sheet with parchment paper.

Using a pastry cutter, cut rings from the puff pastry. For half of these rings, cut a smaller hole in the center. Place them on top of the whole rings, pressing gently to adhere.

Arrange the puff pastry rings on the prepared baking sheet. Brush with the egg yolk and milk mixture.

Bake for about 20 minutes until golden brown. Allow them to cool completely.

In a saucepan, heat most of the milk over medium heat until it is warm and fragrant but not boiling.

In a bowl, mix the remaining milk with the 3 egg yolks, 50 g sugar, 20 g cornstarch, 15 g all purpose flour, and a pinch of salt until smooth.

Gradually pour the hot milk into this mixture while stirring constantly to temper the eggs.

Return this mixture to the saucepan and cook over medium heat until thickened, stirring continuously to a custard that coats the back of a spoon.

Stir in 1 tsp vanilla extract and 1 tsp butter before removing from heat. Cover the cream and allow it to cool completely.

Once cooled, spoon the vanilla cream into each pastry ring, crown with fresh blueberries on top, and dust lightly with powdered sugar before serving.