First thing first, lets sift all the dry ingredients atleast 4 times. So mix jowar flour, baking powder, baking soda, salt together and sift. Put the over on preheat at this time.

Next in another deep bowl mash the bananas, add jaggery powder, vanilla essence and oil/ghee and mix it well, till all are uniformly mixed together. Prepare the baking tin with either a parchment paper or greasing and dusting it with the flour.

Gradually mix the sifted flour one tbsp at time as you fold all of it together with a spatula. This part is important if you want the cake to rise well. 

It might get till thick so add milk 1 tbsp at a time too to  make it easy enough to fold in together.

Remember not to over do the folding too, at the end add all the walnuts and fold it too.

Pour the batter in the tray and tap it to evenly distribute and bake it for 30 mins at 180 degrees. After 30 mins, do a toothpick test and if it comes clean let the cake rest in over for another 15 mins, otherwise bake for another 5 mins.

 After you take if out from the over , let it cool down more before you cut it into small bite size pieces.