Preheat oven to 170 degree C. Line a loaf pan with parchment paper, grease with oil and set aside.
Take banana puree, sugar and vanilla and beat till mixed
Add in eggs and beat till creamy. Add oil, milk and beat again.
Add in flour, baking soda and fold gently. Once it is combined, set aside.
Take cocoa powder in a bowl, add water and mix to form a paste.
Add half of the batter into the cocoa mix and mix well.
Now take the loaf pan, Spoon the plain batter and chocolate batter alternately into the prepared loaf pan. Swirl the batters together with a knife (don't overdo it or the colors will get muddled -- up and back 4-5 times should suffice). No bake for 1 hour and 15 mins. Cover the top of the cake with a foil if the cake is getting too dark.
Insert a toothpick to check whether the cake is done, if it comes out clean then it is done.
Remove it from oven, cool it down for 15 mins.
Invert it on a cooling rack and let it cool completely.
Slice and serve.