In a small bowl, cream butter and 6 tablespoons sugar. Beat in egg and vanilla. Stir in banana and sour cream. Combine the flour, baking powder, baking soda and salt; gradually add to creamed mixture. Combine the walnuts, cinnamon and remaining sugar.
Spoon half of batter into a greased 6-cup fluted tube pan. Sprinkle with nut mixture; top with remaining batter. Bake at 350° for 32-38 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack.