Preheat the oven to 180C/350F/gas mark 4
Use a 20cm/8 inch round cake tin.
Grease and line the cake tin. Measure the flour and baking powder into a large bowl and stir in the sugar. Add the nuts, carrot and banana, and mix lightly. Make a well in the centre and add the sour cream and oil. Beat well until blended. Turn into the tin and bake in the oven for about 1 1/4 hours until the cake is golden brown, and is shrinking slightly from the sides of the tin. A warm knife pushed into the centre should come out clean. Turn out, remove the paper, and leave to cool on a wire rack.
Topping: Measure all the ingredients together in a bowl and beat well until blended and smooth. Spread over the cake and rough up with a fork. Leave in a cool place to harden slightly before serving. Serve cut into thin wedges.
