Defrost the puff pastry. Place the pastry sheets on top of each other and roll out on a floured surface to a 30 x 30 cm sheet, cut nine 10 cm squares from the sheet and place on a baking tray lined with parchment paper. Mix egg yolk with milk, brush squares with it.
Prepare crumbles from flour, sugar and butter and spread evenly on the pastry sheets. Place the tray in the preheated oven. E: middle. T: 225 °C /1 5 to 20 min.
Cool crispy pastry on a cake rack and carefully cut a lid from each pastry piece into small pieces. Whip cream with sifted powdered sugar and cream stiffener until stiff. Fold in vanilla pudding. Spread cream evenly on bottoms of pastry pieces and place lids on top.
Sprinkle with powdered sugar.