Preheat the oven to 350 degrees. Coat a loaf pan with cooking spray.
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.
Mix the flour, sugar, cocoa powder, baking powder, and salt in a large bowl.
Beat the butter with a hand mixer (or stand mixer) in a bowl, then mix in the melted chocolate, eggs, bananas, and vanilla.
Stir in the dry ingredients until just combined; do not overmix.
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 the pan from the oven and let it cool for at least 15 minutes before removing it from the loaf pan. Continue to cool on a wire cooling rack.
Once the bread has cooled completely, sprinkle the top with powdered sugar. Slice and serve. Enjoy.