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.
Whisk in sugars until incorporated.
Add egg and vanilla, and whisk until well-combined.
Stir in flour, baking soda, salt, and cinnamon until combined.
Add chocolate chips and stir in just until combined.
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.
About 3 very ripe bananas, peeled
The butter should just barely be melted. I melt it about 90% of the way and then whisk it until its completely melted.
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.
I like to use semisweet chocolate chips. However, you can use bittersweet, dark, milk, or even white chocolate chips.