Rinse and then soak the whole urad dhal in water sufficient to cover the dhal, overnight.
Next morning, drain the water and wash the soaked dhal 2-3 times, drain again and grind the dhal with the cumin seeds into a paste, using as little water as possible. The lesser the water, the quicker the badi will dry and the batter will have the consistency required for shaping it. Add salt and hing powder and run the mixie once more so that they spread evenly in the batter. The batter may be coarse however the cumin should have ground completely.
Transfer the batter to a bowl and beat it and check the consistency. You need to be able to lift spoonfuls of batter to sun dry, but it should not flow easily either. If you are not able to lift a spoonful of batter, add a teaspoon of water at a time, mix, and try again.