Preheat the oven to 350F. Grease a 9x5-inch loaf pan. Set aside.
In a large bowl, whisk mashed banana and melted butter together until well-combined.
1 ½ cups mashed banana, ½ cup butter
Whisk in sugars until incorporated.
½ cup brown sugar, ½ cup granulated sugar
Add egg and vanilla, and whisk until well-combined.
2 large eggs, 1 teaspoon vanilla extract
Stir in flour, cocoa powder, baking soda, salt until combined.
1 ½ cups all-purpose flour, ½ cup unsweetened cocoa powder, 1 teaspoon baking soda, ¼ teaspoon salt
Add chocolate chips and stir in just until combined.
1 cup chocolate chips
Pour the batter into your prepared loaf pan, smoothing as needed.
Bake for 45-65 minutes, or until a wooden toothpick inserted into the center comes out clean. It's ok if there's melted chocolate on the toothpick, but you don't want batter on it. I check the bread around 35 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
Bananas: You'll need about 3 very ripe bananas, peeled
Butter: The butter should just barely be melted. I melt it about 90% of the way and then whisk it until its completely melted.
Flour: To properly measure the flour, stir or sift it to break it up. Then lightly spoon it into the measuring up and level it off.
Chocolate chips: I like to use semisweet chocolate chips. However, you can use bittersweet, dark, milk, or even white chocolate chips.