In a salad bowl, mash the Roquefort with a fork together with the butter and flour and mix until you get a smooth paste. Cut out four rectangles from the puff-pastry dough. In the center of each rectangle, place one fourth of the Roquefort filling, fold over, and glue shut with egg yolk with a little water mixed in to help it stick. Make sure that the edges are well glued. Arrange your pastries on the sheet of parchment paper and bake (15 min) in the oven at 200°C, making sure not to overbake. Serve immediately.