Mix the two flours in a bowl, then stir in the yogurt and 450ml water. Add the chilli, ginger and roasted cumin and mix. Adjust the consistency by adding more water, if necessary, then stir in the bicarbonate of soda.
Lightly grease the banana leaf pieces. Heat a non-stick tawa, then spray on a little oil.
Pour some batter on to a banana leaf square and place on the hot tawa. Cover with another banana leaf (oily side down) and cook. Once the bottom banana leaf shows little brown specks on it, flip the panki and cook the other side.
You can make a half-moon-shaped panki by putting the batter on one side of the leaf and folding the other side over. Make the other pankis in the same way and serve in the leaves.