Preheat the oven to 350F. Grease an 8x4-inch loaf pan. Set aside.
In a large bowl, whisk mashed banana and melted butter together until well-combined.
Whisk in sugars until incorporated.
Add egg and vanilla, and whisk until well-combined.
Stir in flour, baking soda, and salt until combined.
Remove 1 cup of batter to a separate mixing bowl.
Whisk in cocoa powder and milk until combined.
Pour 1/4 of the plain batter into the prepared pan.
Top the batter with 2 tablespoons chocolate batter.
Continue alternating batters on top of each other until you've used up both batters.
Give the pan a couple of gentle taps but don't stir.
Bake for 40-60 minutes, or until a wooden toothpick inserted into the center comes out clean. I check the bread around 30 minutes and tent with foil if it is browning too quickly.
Let cool in pan for a 5-10 minutes.
Remove bread to wire cooling rack, and let cool completely before slicing.
Notes
The butter should just barely be melted. I melt it about 80% of the way and then whisk it until its completely melted.
About 2 very ripe bananas, peeled
To properly measure the flour, stir it to break it up. Then lightly spoon it into the measuring up and level it off.