Mix the warm water with the yeast and sugar until the yeast is dissolved. Cover and let stand in a warm place for 10 minutes.
Sift flour and salt three times into a large bowl. Add the yeast mixture, half of the ghee and all the curd. Mix into a soft dough, then knead on a floured surface for about 5 minutes or until dough is smooth and elastic. Place the dough in a large greased bowl, cover and let stand in a warm place for 1 1/2 hours or until the dough is doubled in size.
Punch down dough, then knead for 5 minutes. Divide dough into 6 pieces. Roll each piece out into 20cm round naans.
Cover a baking tray with foil and grease the foil. Brush the naan with a little of the remaining ghee and sprinkle with some of the kalonji. Cook naan one at a time under a very hot tandoor or grill for about 2 minutes on each side or until puffed and just browned.