Preheat oven to 180°C.
Peel the banana and slice it into four.
Heat butter, jaggery and water until everything melts together.
Pour this into the loaf tin and place the bananas cut side down on the caramel.
Let it cool down.
In another bowl, add melted butter, jaggery, milk, vanilla essence and oil and mix until combined.
Add the wheat flour, baking powder and cocoa powder and mix to form a thick batter.
Pour the batter over the bananas and smooth out the top.
Bake in preheated oven for 20 minutes.
Allow the cake to cool for 15 minutes.
Loosen the sides of the cake with a spatula and flip it onto a serving plate.
Slice it up and serve.