Dry roast all the spices in low flame, when aroma comes, turn off the heat. Cool it down & ground the spices into powdered biryani masala.
Take 4 medium potatoes/aloo(cut into halves), smeared with salt & turmeric, cook it in pressure cooker with 4-5 cups of water with 2 whistles.
Once the potatoes completely cool down, fry the potatoes in 1/2 cup ghee + 1/2 cup oil until it gets golden brown in colour.
Marinade the chicken(about 1 kg200 gm) with curd, kashmiri chilli powder, salt, ginger-garlic paste, biryani masala, lemon & birista. Marinade it for 45 mins to 1 hr.
Cook the marinated chicken for 10 mins( each side) by covering the pan. Dry off its gravy.
Wash the rice(1 kg long grain fine basmati/dehradun rice) for 5-6 times, soak for 30 mins.
Cook the rice on high flame with water, salt, refined oil, bay leaves, green cardamoms, cloves & cinnamon sticks, do not cook it more than 80%.
Take 4 tbsp lukewarm milk, soak a pinch of saffron strands, add 1 1/2 tsp of kewra water and 3 drops of rose essence & 8-9 drops of mitha attar. Mix it well.
In a thick bottom vessel, spread bay leaves, then 1/2 - 3/4 inch thick layer of rice, chicken pieces, aloo, gravy, biryani masala, saffron milk, ghee, salt & beresta. Continue this method.
Lastly cover the last layer with an ample amount of rice, by sprinkling with 3-4 tsp of ghee, a pinch of biryani masala, add salt and the hard-boiled eggs on the top, close the lid.
Put the biryani vessel on the lowest heat of the gas oven. Place another pan on its top full of hot water. Put it on dum maximum of 30-45 mins to be perfectly done.
Kolkata style chicken dum biryani is ready, best to serve it with any Bengali style chicken or muton chaap or reazala or just with salads.