For a quicker and easier version, use good-quality frozen puff pastry instead of making your own.

1. Place the sifted flour on a work surface; make a well in the centre; add the butter and blend it in roughly with your fingers, leaving the butter in fairly large pieces; quickly mix in the cold water to form the dough; let rest for an hour or two in a cool place;
2. flour the work surface; with a rolling pin, roll the dough out into a large even square about 5 mm (1/4") thick; fold in three like a letter;
3. roll the dough out in the opposite direction; repeat the folding and rolling process several times to create the puff pastry; let rest in a cool place;
4. roll out into an even square about 5 mm (1/4" thick); cut into 8 cm squares; make a cut from each corner about half way to the centre; place a little of the prune or plum filling in the centre and fold every second section into the centre to form a windmill shape;
5. brush with egg yolk; place the pastries on a buttered baking sheet and bake in a preheated oven (220° C or 425° F) for 18-20 minutes until the pastry is puffed and golden.