Defrost puff pastry sheets. Cut once in the middle and then the pieces diagonally (makes 12 pieces). Place on a baking tray covered with baking paper. Brush pieces with egg yolk and sprinkle with brown sugar. Bake pieces of puff pastry in a preheated oven at 200 ° C for approx. 10-15 minutes until golden. Let cool down.
In the meantime, mix the quark, lemon juice and sugar. Fold in whipped cream. Shortly before serving, cut the puff pastry triangles crosswise, brush the base with 1 teaspoon of plum jam, put a dollop of quark cream on top and put the sugar lid on top. Serve immediately.