Preheat your oven to 350°F (175°C). This ensures an evenly cooked loaf.
In a bowl, mash the ripe bananas until smooth. Mix in the melted butter until combined.
Stir in the baking soda and salt. Incorporate the sugar, beaten egg, and vanilla extract until just blended.
Gently fold in the flour, taking care not to overmix.
Separate a little over half of the batter into another bowl and add the cocoa powder. Mix until smooth.
Grease a loaf pan (9x5 inches) and begin layering: spoon some banana batter into the pan, followed by spoonfuls of the chocolate batter. Alternate until both batters are used. Tapping the pan lightly on the countertop will help the batter settle.
Using a knife, swirl through the batter gently to create the marbled effect. Don’t overdo it—just a few swirls will do!
Bake for about 60 minutes, or until a toothpick inserted into the center comes out clean.
Allow to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing. Enjoy!