Preheat the oven to 350 degrees. Coat a loaf pan with cooking spray.
Mix together the flour, sugar, cocoa powder, baking powder, and salt in a large bowl.
Place some water into a pot over medium heat. Place a glass bowl in the pot (making a double boiler) then add the pieces of bittersweet chocolate. Stir often until just melted. Remove from the stove and set aside.
In another bowl, cream the butter until lightened, then beat in the melted chocolate, eggs, bananas, and vanilla. Stir in the dry ingredients until just combined; do not over mix.
Pour the batter into the prepared loaf pan and bake until a tester inserted into the center of the bread comes out almost clean, 60-70 minutes.
Remove from the oven and let cool for at least 15 minutes before removing the bread from the loaf pan. Continue to cool on a baking rack. Once the bread has cooled completely, sprinkle the top with powdered sugar. Slice and serve. Enjoy.