Take a cast iron skillet or tava. Dry roast all the ingredients separately and make a powder of all dry roasted spices. This mixed spices is known as Biryani Masala.
In a large bowl beat hung curd, mix all ingredients (except salt and mutton) to make a smooth paste. Add mutton and salt and keep aside for 4-5 hrs or at least 1 hr..
Wash and clean the rice and soak for 30 mins. Make a pouch of muslin with all the ingredients of rice except rice, salt, ghee and bay leaves and seal. Bring water to boil with bay leaves, salt, ghee and spice-bag in a large pot, add rice and cook until rice is half done.
Drain water and spread the rice on a flat surface. Discard the bay leaves and spice-bag.
Heat 3 tbsp of ghee in a wok and fry thinly sliced onion till golden brown.
Add the mutton and continue frying on medium flame.
I love to add potatoes and egg here at this point. It will make the potatoes juicier and also gives a flavor to the eggs.
Fry them until potato and mutton are almost done.
Soak the saffron strands in warm milk and keep aside for 5 mins. Rub the strands in milk to dissolve them completely. Add rose essence and kewra water and keep aside again for 15 mins.
Now take a large pot and arrange the rice, gravy, mutton, potato and eggs in layers separated by ghee and saffron milk only.
Pick an option from above detailed recipe section for instructions and the follow the same. Once done transfer to the serving plates and serve immediately.
Recipe by Scratching Canvas at http://www.scratchingcanvas.com/kolkata-style-mutton-biryani-step-by-step-recipe/