Preheat your oven to 350°F (175°C). Prepare your 9" x 5" loaf pan by greasing it with butter or lining it with parchment paper.

In a medium bowl, whisk together the flour, cinnamon, baking powder, baking soda, and salt. Set aside.

Cream the unsalted butter, brown sugar, and peanut butter in a large bowl until light and fluffy, about 2-3 minutes.

Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.

Gradually add the dry ingredients to the wet mixture, alternating with the sour cream, mixing on low speed.

Fold in the mashed bananas and chopped nuts gently using a spatula.

Pour the batter into the prepared loaf pan and smooth the top with a spatula.

Bake in the preheated oven for 50-55 minutes until golden brown and a toothpick inserted comes out clean.

Let the bread cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.