Take a medium size bowl ,add besan, cornstarch ,rice flour, salt ,hing powder and chilli powder to it.Mix them well with the spoon.Add water (⅔-3/4 cup) a little at a time and make a slightly flowing smooth batter. The batter should neither be too thick nor too thin. Let it rest for 10 minutes.
Now add curd ,crushed corn kennel, onion,chilli ,coriander and a pinch of baking soda to the batter and mix them together.
Put your appa maker on medium high flame. Brush it nicely with the oil.Once it get medium hot ,pour 1 spoon of batter in each round mould. Cover it with a lid and put on low medium flame and cook for 4 to 5 minutes.
Now turn the corn balls upside down and again let it cook for 2 to 3 minutes.
Once your corn balls are cooked ,turn off the gas and take them out.
Serve hot with coriander chutney (dip).