stir together all dry ingredients except the cocoa powder.
mix the cocoa powder and 1/4 cup boiling water in a small bowl and set aside.
combine the wet and dry ingredients to make one batter. divide the batter in half. add the cocoa to one of the batters (so you now have one chocolate batter and one original banana batter).
scoop a layer of plain batter into your loaf pan, followed by a layer of chocolate. repeat until all batter is used up. take a spoon and swirl the batters together for just a few seconds so you create that pretty marble look!
bake for 50 minutes. you may have to turn the oven 