1 Stick (8 tbsp) unsalted butter, at room temperature
¾ cup sugar
½ cup (packed) light brown sugar
2 large eggs
2 ripe bananas, mashed
¾ cup buttermilk
3 oz bittersweet chocolate, coarsely chopped, or ½ cup store-bought chocolate chips
Preheat the oven to 350?F (180ºC).
Butter a 9 x 5 inch loaf pan and place it on an insulated baking sheet or on two regular baking sheets stacked one on top of the other.
Sift together the flour, cocoa, baking powder, salt and baking soda.
Working with a stand mixer, preferable fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the butter at medium speed for about a minute, until softened.
Add the sugars and beat for 2 minutes more. Add the eggs one at a time, beating for a minute after each addition. At this point, the batter may look a little curdled-it’s okay.
Reduce the mixer speed to low and mix in the mashed bananas. Add the dry ingredients in 3 additions, mixing only until they disappear into the batter. Still on low speed, add the buttermilk, mixing until it is incorporated. Stir in the chopped chocolate. Scrape the batter into the pan.
Bake for 30 minutes. Cover the bread loosely with a foil tent to keep the top from getting too dark, and continue to bake for another 40 to 45 minutes (total baking time is between 70 to 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 at room temperature right side up.