01 - Preheat 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 softened butter with granulated sugar until light and fluffy.
04 - Beat in eggs one at a time, then stir in mashed bananas and vanilla extract until well combined.
05 - Add dry ingredients to wet mixture and gently mix until just combined. Do not overmix.
06 - Fold in chopped walnuts until evenly distributed.
07 - In a small bowl, combine brown sugar and ground cinnamon for the swirl.
08 - Pour half the batter into prepared pan. Sprinkle half cinnamon-sugar mixture evenly over batter. Add remaining batter, then sprinkle remaining cinnamon-sugar. Gently swirl a knife through batter to create marbled effect.
09 - Bake for 50-60 minutes, or until toothpick inserted into center comes out clean.
10 - Let bread cool in pan for 10 minutes, then remove and cool completely on wire rack before slicing.