Soak Saffron in warm milk for 10 minutes. The milk will turn light yellow color. You can also add few drops of food color in the milk.
Take large pot and grease it with ghee.
Now add place one third of the cooked chicken in the bottom and then spread a layer of rice, fried onions (beresta), few potato halves and boiled egg.
Add little saffron milk in this layer.
On top of this layer, add similar layer by adding cooked chicken, fried onions, rice, potatoes, boiled egg and saffron milk.
Finally on the top layer add kewra water and meetha attar.
Now close the lid of the pot with a lid and seal the edges with wheat flour dough.
Cook on high flame for 2 mins. Remove the pot from the stove.
Now place an iron skillet on gas stove and keep the biryani pot on it.
Keep the gas on low flame and allow to heat it for 10 minutes or so.
Turn off the heat and give a standing time of 10 minutes.
Next unseal the edges and serve hot kolkata style chicken biryani with raita.