Source: Adapted from Dorie Greenspan, Baking from My Home to Yours
2 cups all-purpose flour
1 cup unsweetened cocoa powder
1-1/2 teaspo ons baking powder
1/2 teaspoon salt
1/4 teaspoon baking powder
1 stick unsalted butter, at room temperature
3/4 cup sugar
1/2 cup packed brown sugar
2 large eggs
2 ripe bananas, mashed
3 ounces bittersweet chocolate, coarsely chopped (I used 2 oz. milk chocolate, and 1 oz. dark chocolate)
Place an oven rack in the center position and preheat the oven to 350 degrees. Butter a 9x5 inch loaf pan (I used Pam for Baking spray) and place on an insulated baking sheet. This will keep the bottom of the loaf from overcooking.
Sift the flour through baking soda together in a large bowl.
Using a stand mixer with a paddle attachment, beat the butter at medium speed for a minute or until softened. Add in the sugars and beat for another 2 minutes. Add the egss, one at a time, and beat for another minute after each addition. The batter may look curdled, but that's ok. Reduce the mixer speed to low and mix in the bananas. Now, add the dry ingredients in 3 additions and mix until they are just mixed in. While on low speed, add in the buttermilk and mix until incorporated. Stir in the chopped chocolate. Scrape the batter into the prepared pan.
Bake for 30 minutes, then cover the bread loosely with foil tented over it. Then bake for an additional 40-45 minutes, or until a thin knife inserted comes out clean. Transfer the pan to a wire rack and allow to cool for 20 minutes. Then unmold and allow to cool to room temperature.