Fry the chicken in the oil until they start to brown. Remove and set aside.
Fry the onions, garlic and ginger for 2 minutes before adding the spices. Allow to fry for another minute and add the chicken back into the pot.
Stir to coat the chicken in all the spices and add the tomatoes, sugar and chicken stock.
Lower the heat and allow to simmer for 15 minutes.
Heat 1/2 cup of oil in a large frying pan and fry the potatoes until golden and crisp. Season with salt and set aside.
Fry the onions in the same oil until crisp and golden and then drain and set aside.
To make the spiced butter, melt the butter, turmeric and salt together.
To assemble the dish, layer a ½ of the rice at the bottom of a greased oven-proof dish. Drizzle with half of the spiced butter. Next, cover the rice with the chicken curry. Follow with the rest of the rice and the remaining butter. Finally, cover the rice with the fried potatoes and onions (reserve some onions for serving). Pour over the chicken stock and cover with foil (or a lid if your oven-dish has one).
Place in the oven for 25-30 minutes.
To make the raita, combine all the ingredients in a bowl and mix well.
Remove the biryani from the oven and top with the crispy onions.
Serve the Biryani with the Buttermilk Raita and fresh Coriander/cilantro.