(Optional) If you don't have buttermilk, then prepare buttermilk by adding 1 1/2 tsp lemon juice and add enough milk to fill 1/2 cup in the measuring bowl (approx 1/2 cup milk). Mix it and keep it aside, buttermilk is ready.
Preheat oven to 350 degrees F (180 degrees Celsius). Meanwhile, mash the bananas in a bowl until smooth.
Add the melted butter, buttermilk, vanilla extract and jaggery powder to the mashed bananas and mix it thoroughly.
Using a sieve, sift whole wheat flour, unsweetened cocoa powder, baking soda, baking powder and salt, over the wet ingredients mixture.
Gently fold dry ingredients with the wet ingredients mixture using a spatula. Do not over-mix at this stage. Finally add the half cup of chocolate chips, and fold them to the cake batter.
Grease a standard loaf pan (8 1/2 x 4 1/2 inches) with some oil (or butter) and dust with whole wheat flour. Pour batter into greased and dusted loaf pan.
Optionally, sprinkle the extra 2 tablespoon of chocolate chips on top (Check ** Notes down below for a pro tip).
Bake in the preheated oven for around 55-60 minutes or till the cake is done (when the inserted toothpick comes out clean).
Remove the cake from the oven and allow it to cool before slicing and serving. You can store it in the refrigerator in an airtight container and use within a week.