Preheat the oven to 180C/350F/gas mark 4
Use a 20cm/8 inch deep round cake tin.
Grease and line the cake tin. Cream the butter and sugar until light and fluffy. Mash the bananas and take the pulp out of the passion fruit. Stir in with the walnuts, the yoghurt, and the sifted flour.
Spread the mixture into the prepared tin and bake for about 1 hour. Test with a warm knife. Cool for 10 minutes before turning on to wire rack to cool.
Icing. Sift icing sugar into small heat proof bowl, stir in butter and enough passion fruit pulp to make a stiff paste. Stir over hot water until the icing is spreadable. Spread over the cooled cake.