Preheat oven to 350°F (175°C). Grease a 9" x 5" loaf pan. (Line it first with parchment paper if you like.)
Whisk together the flour, cinnamon, baking powder, baking soda, and salt. Set aside.
Using an electric mixer on medium speed, beat the butter, brown sugar, and peanut butter until fluffy. Add the eggs, one at a time, mixing well after each addition. Mix in the vanilla.
Reduce mixer speed to low. Add the flour mixture in three portions, alternating with two portions of the sour cream.
Stir in the bananas, followed by the nuts if using.
Spread the batter evenly in the prepared pan. Bake 50 to 55 minutes, or until the bread is golden brown and a pick inserted into the center comes out clean.
Cool in the pan on a wire rack for 10 minutes. Then transfer the bread from the pan directly onto a wire rack to cool completely.
Notes
Store in an airtight container at room temperature up to 3-4 days.