Preheat the oven to 180C/350F/gas mark 4
Use a 20cm/8 inch round cake tin.
Cream the butter and sugar. Add the yoghurt and mix until smooth.
Sift in the flour, baking powder and salt and fold in. Mash the bananas and fold in until evenly blended. Spoon into the greased and floured cake tin and smooth over the surface. Bake for about 40 minutes. Test with a warm knife. Cool for 10 minutes, turn on to a rack and leave until cold.
Slice the cake in half crossways and spread half the whipped cream evenly over the bottom half. Sprinkle the nuts evenly over the cream and then replace the top of the cake. Spread the remaining cream over the top of the cake and, if you wish, decorate with sliced bananas added at the last minute before serving.
