Preheat your oven to 350°F (175°C) and generously grease a 9" x 5" loaf pan.

In a medium bowl, whisk together all-purpose flour, ground cinnamon, baking powder, baking soda, and salt until well combined.

In a large mixing bowl, beat together unsalted butter, light brown sugar, and creamy peanut butter until light and fluffy. Add eggs one at a time, and then blend in vanilla extract.

Gradually add the dry ingredients to the wet mixture, alternating with sour cream and mixing just until combined.

Fold in the mashed bananas and optional chopped nuts until evenly distributed.

Pour the batter into the prepared loaf pan and bake for 50-55 minutes or until a toothpick comes out clean.

Remove from the oven, let it cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.