Place the mashed potatoes in a bowl. Add rock salt, peanuts, sesame seeds, green chilli- ginger paste and one cup sabudana and mix well. Add arrowroot powder and mix. Place the remaining sabudana in a plate. Take some of the mixture and roll into a ball. Place the ball over the sabudana in the plate and press lightly into a tikki. Heat sufficient oil in a kadai and deep fry the tikkis till crisp and golden brown. Drain on absorbent paper and serve hot.