• Preheat your oven to 350°F. Grease or line a standard 9-inch x 5-inch loaf pan and set it aside.

Melt the chocolate.

• In a medium-size, heat-safe bowl, place the chopped chocolate and butter. In the microwave in 30-second bursts, or over a simmering pot of hot water, melt until smooth. Set aside to cool briefly.

Prepare the batter.

• In a large bowl, place the mashed bananas, granulated sugar, sour cream, vanilla and egg, and mix to combine well.
• Add the melted chocolate and butter mixture, and mix again until well-combined.
• Add the cocoa powder, flour, xanthan gum, baking powder, baking soda and salt, beating to combine. Do not overmix. The batter will be very thick.
• Add the chocolate chips, and mix until they’re evenly distributed throughout the batter. Add the optional chopped banana and fold in the banana as gently as you can to avoid mashing the extra banana.
• Transfer the batter to the prepared baking pan, and press into an even layer in the pan. Again, the batter will be very thick.
• Smooth the top with a wet silicone spatula or wet hands. For a uniform rise, use a small, sharp knife to slash the center of the batter from one short side to another, about 1/4-inch deep.

Bake the banana bread.

• Place the loaf pan in the center of the preheated oven and bake for 30 minutes.
• Rotate 180° and continue to bake until the loaf is firm to the touch on top and a toothpick inserted in the center comes out clean, about another 25 minutes.
• If you’ve added the additional chopped banana, the baking time will likely need to be increased by about 5 or 10 minutes.
• Remove from the oven and allow to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.
• Slice and serve.