In a large skillet, fry potatoes in 2 tbsp of vegetable oil (or ghee) until brown. Drain off the fat and set the potatoes aside. Add
remaining 2 tbsp of oil to the skillet and fry onions finely
chopped, garlic cloves minced and fresh ginger minced until onion is soft and golden. Add chili, pepper, turmeric, cumin, salt and tomatoes. Fry, stirring constantly for 5 minutes. Add yogurt, mint, cardamom and 5 cm cinnamon stick. Cover and cook over a low heat, stirring occasionally until the tomatoes are cooked to a pulp. Add a little hot water if necessary to avoid the mixture becoming too dry and sticking to the bottom of the pan.
When the mixture is thick and smooth, add the chicken pieces and stir well to coat them with the spice mixture. Cover and cook over a very low heat until the chicken is tender, approximately 40 minutes. There should only be a little very thick gravy left when the chicken is finished cooking. If necessary cook uncovered for a few minutes to reduce the gravy.
Wash rice well and drain in colander for at least 30 minutes.
In a large skillet, heat 2 ½ tbsp vegetable oil (or ghee) and fry the large onion diced until golden. Add saffron, cardamom, 3 whole cloves, 2.5 cm cinnamon stick, ginger ground and rice. Stir continuously until the rice is coated with the spices.
In a medium-size pan, heat the chicken stock and salt. When the stock is hot pour over the rice and stir well. Gently stir the chicken mixture and potatoes into the rice and bring to the boil. Cover the saucepan tightly, turn heat to very low and steam for 20 minutes. Do not lift lid or stir while cooking.
Spoon biryani onto a large warm dish and serve.