Heat oven to 350°
Spray a 9x5 inch loaf pan with nonstick baking spray.
Mash bananas into the bottom of a large bowl. Whisk in melted butter, rum, brown sugar, egg and vanilla.
Place baking soda, salt, flour and cocoa powder in a sifter and sift into wet ingredients. Stir with a spoon until combined.
Add chocolate chunks and stir to blend.
Pour into prepared pan and bake for 60 - 65 minutes, until a toothpick inserted into the center of cake comes out clean.
Cool in pan for 15 minutes, then take out and place on a cooling rack.