Preheat oven at 170 degree C and line a cake tin with butter paper and keep aside.
Sieve wholewheat flour, baking powder and baking soda together and keep aside.
Peel the overripe bananas and take in a bowl and mash them using a fork until they are smooth.
In another bowl take oil and grated jaggery and mix well.
Beat the curd in another bowl and add to the wet mixture. Mix it well.
Add the mashed banana into the wet mixture and mix it well.
Now it is time to fold the dry ingredients into wet ingredients. At this moment, if you see the batter is too dry, add milk slowly and fold it.
Add 1 tablespoon of crush walnuts into the batter and mix
Now pour the batter in the lined mold and add the remaining walnuts on top.
Bake it at 170 degree for 40 minutes or until a toothpick comes out clean.
Allow to cool the cake and slice and serve it. You can also store it for 3 to 4 days at room temperature.