Cook the sweet potatoes in boiling water till almost done. Drain and allow them to cool. Peel, cut into wedges and keep aside. Blend cornstarch in half a cup of water to make a thin batter. Grind walnuts to a coarse powder.
Add cinnamon powder, pepper powder, castor sugar and salt and mix well. Spread out in a large plate. Dip potato wedges in cornstarch batter, then roll in walnut mix and refrigerate the same for an hour to give the crust some time to set.
Heat sufficient oil in a kadai and deep-fry the walnut crusted potatoes till golden and crisp. Drain onto an absorbent paper and serve hot.