Remove meat from chicken. Discard skin and bones. Coarsely chop the meat.
Melt butter in a large saucepan. Add onion and leeks. Cook, stirring occasionally, for about 5 minutes, or until leeks are soft. Add flour. Cook, stirring for 1 minute. Gradually add stock, stirring until combined. Add chicken, cream and corn. Simmer, stirring occasionally, for 5 minutes. Remove. Cover and refrigerate until cool. Stir in chives.
Spoon chicken mixture into a pie dish (7-cup capacity). Gently pull pastry sheet to cover dish. Press edge to seal. Trim. Make a slit in top of pie to allow steam to escape. Brush top with milk. Place on an oven tray.
Cook in a hot oven (200°C) for about 45 minutes, or until pastry is golden brown.
Serve pie with mashed potato garnished with chopped chives.
Tip: Decorate top of chicken pie by using a pastry cutter to cut out shapes from pastry scraps. If pastry is browning unevenly during cooking, cover that browned area with a piece of foil.