1Preheat the oven to 398F..
2Quickly rinse cherries in cold water, dry them. Remove pits using olive-pitter, if you have one. Cut the cherries into quarters and put them in a bowl with the hazelnuts and vanilla sugar. Stir to coat the fruit.
3Roll out the puff pastry on your work surface. Using a fluted 10 centimeters cookie cutter, make circles with the dough. Put half the circles on a baking sheet covered with parchment paper. On the other half of the circles cut a small pattern in the center and otherwise make small incisions with a knife. Using a brush, lightly moisten the edge of the circles with water, then put a spoonful of the cherry mixture in the center of the dough. Cover with the second circle of dough, press down firmly with your fingers on the edges, then a fork to seal the two layers of dough to push out the air.
4Stir the egg yolk with a little milk, brush this mixture over the tops of the pies.
5Cook for about 20 minutes to get them nicely browned, let cool before serving them. If desired, sprinkle them with powdered sugar.