Mix quark with egg yolk, sugar, vanilla extract, lemon juice, and cornstarch until smooth.
Roll out the puff pastry and cut it into 12 equal squares. Place 1 heaping tablespoon of quark filling on 6 of the squares. Take the remaining 6 squares and cut diagonal, closely spaced slits in the center. Lay the cut squares over the filled squares, press the edges firmly, and seal them.
Place the quark pastries on a baking sheet lined with parchment paper, brush with egg yolk, and bake in a preheated oven at 200 °C (390 °F) top/bottom heat for about 18–20 minutes until golden brown.
Let them cool and dust with powdered sugar.
