Preheat the oven to 350F (176C). Spray a 9x5-inch loaf pan with cooking spray and line with parchment paper.
In a large bowl, add the bananas. Mash with a fork until smooth.
Stir in the cooled melted butter until smooth.
Add the brown sugar, sour cream, eggs, and pure vanilla extract to the bowl. Stir until combined.
Add the gluten-free flour blend, baking powder, baking soda, ground cinnamon, and salt and xanthan gum (if using) to the bowl. Stir until no more streaks of flour remain.
Stir in the chocolate chips.
Spoon the batter into the prepared loaf pan. Slice the remaining banana in half lengthwise. Place the two banana halves on top of the batter.
Bake at 350F (176C) for 60-70 minutes on the center rack of your oven, until a toothpick in the center comes out clean. If the banana bread is browning too quickly, cover it with foil. The internal temperature of your banana bread should be about 205F (121C). Let cool on a wire rack before slicing.