Preheat the oven to 350 degrees. Spray a 9 inch by 5 inch loaf pan with cooking oil spray. Set aside.
Combine the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment. Mix on medium speed until the mixture looks light and fluffy. Add one egg at a time, beating well each time. Add the vanilla to the mixture and mix well on medium speed until smooth.
In a small bowl, sift together the flour, baking powder and salt. With the mixer running on its lowest speed, add the flour mixture to the bowl little by little, until just combined. Remove 1 cup of batter from the mixer to a large mixing bowl. Add the mashed bananas to the mixer and mix well until smooth. Pour into the prepared pan.
Stir the cocoa powder and oil into the batter in the bowl until evenly distributed. Pour into the prepared pan. Drag a knife through the batter a few times.
Slide the pan into the preheated oven and cook for 45-50 minutes. The bread is done when a toothpick inserted in the center comes out clean.
Let cool before slicing and serving.