Beat butter and sugar until light and creamy. Add lightly beaten egg, beat well. Stir in combined peeled, mashed banana and lemon juice. Fold in sifted dry ingredients alternately with yoghurt. Stir in half the chopped walnuts, mix until smooth. Spoon mixture into greased and greased paper lined 25 cm x 8 cm (10in x 3 in) bar tin, sprinkle with remaining walnuts.
Bake in moderate oven 40 minutes or until cooked when tested. Allow cake to cool 5 minutes in tin, turn out and cool on wire rack.
To make the icing. Beat the three ingredients together until smooth and delicious. If it is too runny, add more icing until it is at the consistency you require. E.