Preheat the oven to 350°F (180°C) and center the rack.
Butter a 9×5-inch (23cm x 12.5cm) loaf pan and place it on two stacked baking sheets (to prevent the bottom of the bread from burning).

Combine the flour, cocoa powder, baking powder, salt, and baking soda. Set aside.
Using an electric mixer, beat the butter with the sugar and brown sugar for 2 minutes.
Add the eggs one at a time, beating well between each addition.

Reduce the mixer speed and add the mashed bananas, prepared dry ingredients, and buttermilk in 3 alternating additions, beginning and ending with the dry ingredients.
Mix until the batter is smooth.
Fold in the chopped chocolate and mix.

Pour the batter into the prepared loaf pan and bake for 30 minutes.
Cover the bread with aluminum foil and continue baking for 40-45 minutes, or until a toothpick inserted into the center comes out clean.
Total baking time: 70-75 minutes.

Remove from the oven and transfer to a wire rack to cool for 20 minutes.
Remove from the pan and let cool completely.