Preheat the oven to 350°F (175°C) and grease a 9x5-inch loaf pan.
In a large mixing bowl, mash the bananas until smooth. Stir in the melted butter and mix well.
Add the sugar, eggs, and vanilla extract. Whisk until fully combined.
In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon (if using).
Gradually fold the dry ingredients into the wet ingredients, mixing gently until just combined. Avoid overmixing.
Stir in the sour cream (or Greek yogurt) for extra moisture. If adding walnuts or chocolate chips, fold them in at this stage.
Pour the batter into the prepared loaf pan and smooth the top.
Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean.
Let the banana bread cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
