Take a small pot & keep some pieces of burning koyla (coal) into it.
Keep this pot in the middle of the biryani vessel and put 2 tsp ghee over the burning koyla. Cover tightly with lid. Keep weight on the lid or use maida to seal the vessel.
:- Do Not allow the smoke to go outside of the vessel.
Keep the biryani on a very low flame for 5 minutes.
Remove the lid and remove the pot from the vessel.