Preheat oven to 350F. Generously butter and flour a 10-inch tube pan or Bundt pan.
To prepare cake, combine flour, baking soda and salt in a medium bowl.
Combine butter and sugar; beat with an electric mixer about 4 minutes. Add eggs one at a time, beating well after each addition. Beat in banana and vanilla. Add flour mixture and buttermilk alternately, beginning and ending with flour mixture and mixing until just combined. Fold in walnuts.
Pour batter into prepared pan and smooth top. Bake 45 to 50 minutes. Let cool in pan on wire rack 10 minutes. Remove from pan. Let cool completely.
To prepare glaze, whisk together powdered sugar and cocoa powder. Slowly stir in milk and vanilla. Whisk until smooth. Add a bit more milk to thin glaze, if needed. Spread over cake. Serves 12.