1. Preheat the oven to 350°F (175°C) and grease or line a 9×5-inch loaf pan.

2. In a medium bowl, mash the bananas until smooth.

3. In a large bowl, cream together butter, granulated sugar, and brown sugar until fluffy.

4. Beat in the eggs one at a time, then add vanilla extract.

5. In a separate bowl, whisk together flour, baking soda, salt, and cinnamon.

6. Gradually add the dry ingredients to the wet mixture.

7. Fold in mashed bananas and buttermilk just until combined.

8. Stir in the chopped nuts gently to avoid overmixing.

9. Mix the swirl ingredients (brown sugar and cinnamon) in a small bowl.

10. Pour half of the batter into the prepared pan, sprinkle the swirl mixture, then top with the remaining batter.

11. Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean.

12. Cool for 10 minutes in the pan, then transfer to a wire rack to cool completely.