Preheat oven to 350°.
In a mixing bowl, add bananas, butter, sugar, eggs and vanilla. Beat at medium speed until combined.
Melt chocolate (in a pan over low heat or microwave) and mix into batter.
In another bowl combine flour, cocoa powder, baking powder and salt.
Add to batter and mix for about 2 minutes, until well combined. Stop half way through and scrape down the sides of the bowl.
Spray or butter a 9 x 5 inch loaf pan.
Pour batter into pan, spreading evenly.
Bake for 50 - 60 minutes or until a toothpick inserted in the center comes out clean.
Loosen loaf from pan with a knife and remove to a wire rack to cool.
Dust with powdered sugar.
Slice and serve warm or at room temperature.