Thaw 3 sheets of puff pastry overnight in the refrigerator.

Position racks in the top and bottom thirds of the oven and heat the oven to 375°F.

In a medium bowl, combine the cream cheese, 3 Tbs. of the sugar, the vanilla bean seeds (or extract), and 1 of the egg yolks. Mix with a wooden spoon until well combined and smooth.

Combine the blueberries, 2 Tbs. of the sugar, the cornstarch, crème de cassis, and salt in another medium bowl and mix gently.

On a lightly floured surface, roll each pastry sheet into a 10-inch square. Using a 4-inch round cutter (or a small plate as a guide), cut out 4 rounds from each sheet. Arrange them on 2 parchment-lined rimmed baking sheets.

In a small bowl, beat the remaining egg yolk with 1 tsp. water. Brush the outer edge of each pastry round with the egg wash. Dollop 1/2 Tbs. of the cream cheese mixture in the center of each round. Top with 1 Tbs. of the blueberry mixture. Fold in half to form a half-moon shape and pinch the edges together to seal them. Lightly brush each pie with egg wash and sprinkle with 1/2 tsp. of the remaining sugar. With the tip of a paring knife, cut a steam vent in the center of each pie.

Bake until golden-brown, about 25 minutes, swapping and rotating the baking sheets’ positions about halfway through. Cool slightly on the baking sheets and then transfer to a rack to cool completely. Before serving, sprinkle the pies with confectioners’ sugar.