In a large frying pan add butter & melt over high heat. Add leek & garlic & cook for 3 mins or until leek softens.
Roughly dice chicken (into pieces approx 1cm) and add to pan. Cook over high heat until chicken is browned.
Mix cornflour & stock together and add to chicken mixture. Add cream. Reduce heat to medium & stir for a few minutes or until sauce thickens.
Set aside to allow mixture to cool. To speed up the process, refrigerate mixture or spread out onto a tray.
Preheat oven to 200ºC.
Cut each sheet of pastry in half (giving you 2 large rectangles). Spoon some mixture onto one half of each piece. Fold the pastry over to enclose the filling and pinch edges to seal. I usually fold over the edges to stop mixture from running out. Cut a slit in the top of each one.
Place pastry squares onto lined oven trays and brush the top with beaten egg. Bake in preheated oven for 15 mins or until golden.