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.
⅓ cup salted butter, 1 cup mashed banana
Whisk in sugars until incorporated.
½ cup light brown sugar, ¼ cup granulated sugar
Add egg and vanilla, and whisk until well-combined.
1 large egg, 1 teaspoon vanilla extract
Stir in flour, baking soda, and salt until combined.
1 ½ cups sifted all-purpose flour, 1 teaspoon baking soda, ¼ teaspoon salt
Remove 1 cup of batter to a separate mixing bowl.
Whisk in cocoa powder and milk until combined.
¼ cup unsweetened cocoa powder, 2 tablespoons milk
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 45-60 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
Butter: The butter should just barely be melted. It shouldn't be hot and sizzling. I melt it about 90% of the way and then whisk it until it is completely melted.
Bananas: You'll need about 2 very ripe bananas, peeled. I like to use bananas that are brown and spotted but not black.
Flour: It's important to properly measure the flour in order to avoid heavy or doughy bread. To properly measure the flour, stir it to break it up. Then lightly spoon it into the measuring up and level it off.
Cocoa powder: I use regular unsweetened cocoa powder in this recipe.