Center a rack in the oven and preheat the oven to 350 degrees F. Butter a 9×5 inch loaf pan and place it on an insulated baking sheet or on two regular baking sheets one on top of the other. This extra insulation will keep the bottom of the bread from overbaking.
Sift together the flour, cocoa, baking powder, salt and baking soda.
Working with a stand mixer with a paddle attachment, or with a hand mixer in a large bowl, beat the butter at medium speed for about a minute until soft. Add the sugars and beat for 2 minutes more. Add the egs one at a time, beating for a minute after each addition. At this point , the batter may look a little curdled- it’s ok. Reduce the mixer speed to low and mix in the mashed bananas. Add the dry ingredients in three additions, mixing only until they disappear into the batter. Still on low sweep, add the buttermilk, mixing until it is incorporated. Stir in the chocolate chips. Scrape batter into prepared pan.
Bake for 3o minutes. Cover the bread loosely with a foil tend to keep the top from getting too dark, and continue to bake for another 40-45 minutes (total baking time is between 65 and 75 minutes), or until a thin knife inserted into the center comes out clean. Transfer the pan to a rack and cool for at least 20 minutes before running a knife around the edges of the bread and unmolding it. Invert and cool to room temperature right side up.