1.Grease a deep 20cm round cake pan; line base with paper; grease paper.
2.Cream butter and sugar in small bowl with electric mixer until light and fluffy, add egg, beat until combined. Transfer mixture to large bowl, stir in banana, walnuts and yoghurt, then sifted flours.
3.Spread mixture into prepared pan, bake in moderate oven for about 1 hour. Stand 5 minutes before turning on to wire rack to cool. Top cold cake with icing.
4.To make the passionfruit icing: Sift icing sugar into small heatproof bowl, stir in butter and enough passionfruit pulp to make a stiff paste. Stir over hot water until icing is spreadable.
We used 2 bananas for this recipe.Note