Wash and peel the skin of potato. Cut the head and tail portions slightly and slice into 1/8 inch thick rings. Set aside. Do not slice too thick, otherwise it will not cook properly. Use a sharp knife for slicing.
In a wide mixing bowl, mix all the flours (besan and rice flour), red chilli powder (use Kashmiri chilli powder to avoid food color), ajwain or cumin seeds, salt, cooking soda and food color if using.
Add water gradually to make a slightly watery batter (like dosa batter). If you make it too watery, bajji will have tails; at the same time, thick batter yields hard bajjis. The batter should be thick enough to flow like a ribbon and coat the potato slices well. Dip the potato rings into the batter and coat them evenly.
Heat oil in a kadai and check with a drop of batter to find the correct heating point. If you drop a little batter, it should rise immediately and float on top. As soon as you get this heating point, lower the flame to medium and gently drop the batter-coated potato slices into the hot oil.
Deep fry until the bajji turns golden brown and the bubbles cease. Flip gently and cook for another minute. Drain and remove onto a paper towel.
Enjoy hot, puffy potato bajji with tomato ketchup, coconut chutney and a hot cup of tea!