01 - Preheat the oven to 350°F. Grease and flour a 9x5-inch loaf pan or line with parchment paper.
02 - In a medium bowl, whisk together flour, baking soda, and salt. Set aside.
03 - In a large bowl, cream the softened butter with granulated sugar until light and fluffy.
04 - Beat in the eggs one at a time, then stir in mashed bananas and vanilla extract until well combined.
05 - Add the dry ingredients to the wet mixture and gently mix until just combined—do not overmix.
06 - Fold in the chopped walnuts until evenly distributed.
07 - In a small bowl, combine the brown sugar and cinnamon for the swirl.
08 - Pour half the batter into the prepared loaf pan. Sprinkle half of the cinnamon-sugar mixture evenly over the batter. Add the remaining batter on top, then sprinkle the rest of the cinnamon-sugar. Gently swirl a knife through the batter to create a marbled effect.
09 - Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean.
10 - Let the bread cool in the pan for 10 minutes, then remove and cool completely on a wire rack before slicing.