Preheat the oven to 350°.
Coat a loaf pan with non-stick cooking spray.
Mash bananas in a small bowl. Add the sour cream and vanilla to the bananas and whisk just until blended. Set aside.
Sift the flour, baking soda and baking powder together into a medium bowl. Stir to combine. Set aside.
Place the cocoa powder in the small bowl. Pour the boiling water over the cocoa and stir until it forms a smooth paste.
If it is too thick, add a little more water and stir. Set aside.
Place butter and sugar in a large bowl. Beat on medium speed until the butter is light and combined with sugar.
Add the eggs one at a time and continue beating at medium speed.
Bring mixer speed down to low and gradually begin to add flour.
Add a little banana mixture at a time. Then, continue adding flour and banana mixture until blended.
Transfer half of the batter to the bowl with your cocoa paste and blend it together.
Drop alternating spoonfuls of dark and light batters into the prepared pan.
Marbleize batter by gently swirling batter down the length of the pan. Tap pan gently to even out the batter.
Bake the banana bread for about 60 minutes until a toothpick inserted comes out clean.
Allow to cool before slicing and serving.