Preheat oven to 180°c. Melt butter in a deep pan over medium heat. Add chicken, coarse black pepper and salt. Stir occasionally until excess water/moisture has evaporated and chicken is cooked through. Add sliced mushrooms and leeks and cook for 4-5 minutes. Stir in white pepper powder, white chilli powder, onion powder and garlic powder, cook for 1-2 minutes then reduce heat to low and pour in the double cream. Stir well and simmer for 10 minutes. Transfer filling into a pie dish and cover with ready rolled puff pastry. Trim off excess and design/ crimple edges as desired. Brush with beaten egg and bake in preheated oven for 40-45 minutes until pie is golden brown and crisp.