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