In a sizable bowl, combine chicken pieces with yogurt, ginger-garlic paste, turmeric powder, biryani masala, green chilies, and salt. Stir to coat the chicken, cover, and marinate for at least 1 hour.

Rinse basmati rice under cold water until clear. Soak for 30 minutes. In a pot, boil water, add soaked rice and cumin seeds, parboil for 5-6 minutes until 70% cooked, then drain.

In a heavy-bottomed pot, heat ghee or oil, sauté onions until golden brown (10-12 minutes). Stir in marinated chicken, cook for 8-10 minutes until cooked through.

Layer biryani by spreading half of parboiled rice over chicken, sprinkle with half chopped mint and cilantro, then add remaining rice and herbs. Drizzle saffron milk if using.

Cover tightly and reduce heat to lowest, cook on 'dum' for 25-30 minutes, letting flavors meld and rice finish cooking.