Dry roast cumin seeds, fennel seeds, chana dal, rice, peppercorns, curry leaves and red chillies till fragrant. Set aside to cool slightly.
Put ¼ tsp turmeric powder, salt and tamarind pulp into a mixer jar. Add the roasted spices and a little water and grind into a fine paste.
Peel sweet potatoes and cut them into fingers. Parboil with ¼ tsp turmeric powder and salt. You can also parboil them whole after peeling and then cut into fingers.
Put the ground masala into a large bowl, add the sweet potato fingers, a little salt and mix well so that all the fingers are well coated with the masala. Rest for ½ hour in the refrigerator.
Heat sufficient oil in a non stick pan. Roll the fingers in rawa and shallow fry, turning a few times, till golden on all sides.
Drain on absorbent paper and serve hot.