Preheat oven to 350°. Spray a 9x5-inch loaf pan with baking spray with flour.
In a large bowl, beat butter and sugar with a mixer at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Add banana, beating to combine.
In a medium bowl, whisk together Swans Down® Cake Flour, baking powder, baking soda, and salt. Gradually add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating just until combined after each addition. Stir in walnuts. Pour batter into prepared pan.
Bake until a wooden pick inserted in center comes out clean, about 1 hour. Let cool in pan for 10 minutes. Remove from pan, and let cool completely on a wire rack.
Makes 1 (9x5-inch) loaf