Heat oven to 175 degrees C. In a large bowl and using an electric mixer, cream together butter and sugar. Beat in egg, milk, vanilla, and mashed banana until well blended. Stir in orange zest.
Sift together dry ingredients, stir into creamed mixture until well mixed. Spread into a 20 cm round cake pan.
Bake for 25-30 minutes or until cake springs back when lightly touched.
Top with orange icing or a cream cheese frosting when cake is cold.
Icing: Beat together butter and icing sugar, add sufficient orange juice to make a spreadable icing, mix in orange zest.
Spread evenly over cake with a spatula, sprinkle over crushed walnuts and allow to set.