Stir together first 3 ingredients with a fork in a small bowl. Set aside.
Roll each pastry sheet into a 12- x 8-inch rectangle. Cut each sheet into 6 (4-inch) squares.
Spoon filling evenly into centers of squares. Brush edges of pastry with water. Fold each square of pastry into a triangle, gently pressing edges together with fingers. Place triangles on an ungreased baking sheet, and seal with a fork. Brush pastries with egg yolk.
Bake at 400° for 12 minutes or until golden. Serve immediately.