Preheat oven to 350°F (175°C). Grease a 9x5 inch loaf pan and line with parchment paper.

Mash the bananas thoroughly in a large bowl — you want them very smooth with just a few lumps.

Stir in melted butter, sugar, egg, and vanilla extract until well combined.

Add baking soda, salt, and cinnamon. Stir to combine.

Fold in flour until just incorporated — do not overmix or the bread will be tough.

Fold in the chocolate chunks and walnuts if using. Reserve some chocolate chunks for the top.

Pour batter into prepared loaf pan. Press reserved chocolate chunks on top.

Bake for 55-65 minutes until a toothpick inserted in the center comes out with just a few moist crumbs. Cover with foil in the last 15 minutes if top is browning too quickly.

Cool in pan for 15 minutes, then turn out onto a wire rack. Slice and serve warm or at room temperature.