Move oven rack to position where pans will be in the center of the oven. Preheat oven to 350 degrees. Lightly grease the bottoms of 2 loaf pans using non-stick baking spray or butter and flour.
In the bowl of a stand mixer fitted with the paddle attachment, beat sugar and butter until light and fluffy. Add eggs one at a time, lightly beating until incorporated.
In a medium size mixing bowl, whisk together flour, baking soda and salt, set aside.
Mash bananas on a plate using a fork. Add bananas, buttermilk and vanilla, mix just until incorporated. With mixing speed on low, gradually add in the dry ingredients. Remove bowl from stand and place about ¼ of the batter into a mixing bowl. Combine the warm water and the cocoa powder, add mixture to the removed batter creating a chocolate batter. Pour in mini chocolate chips.
Pour plain banana batter into pans, dividing evenly between the two. Then pour the chocolate batter on top. Gently swirl with a knife or spatula (if you overdo it you wont have nice swirls).
Place in oven and bake for approximately one hour, or until a cake tester inserted into the center comes out clean, and the loaves are golden brown on the top. Remove from oven and let cool for 10 minutes, then loosen the sides of the loves and remove from pans onto wire rack. Let cool completely (about 2 hours) before slicing and eating.