Preheat oven to 350 degrees. Line two 9-inch round cake pans with wax paper.
Sift and measure flour, sift again into large bowl with sugar, baking powder, baking soda and salt. Add butter, banana, and 1/3 cup buttermilk. Beat 2 minutes. Add remaining buttermilk and eggs; beat 2 minutes. Fold in walnuts.
Pour into prepared pans. Bake 30 to 35 minutes or until toothpick inserted in center comes out dry. Cook on racks 15 minutes.
Remove from pans and cool completely before frosting. Frost with Creamy Butter-Cheese Frosting or Banana Butter-Cheese Frosting variation. Makes 12 to 16 servings.
Variation: Banana Sheet Cake: Bake in a 13x9x2-inch buttered and floured baking pan at 350 degrees for 40 minutes.