Preheat the oven to 180C/350F/gas mark 4
Use a 20cm/8 inch ring tin.
Grease and line the ring tin. Cream the butter and sugar in a small bowl until light and fluffy. Add the sour cream and mix well. Stir in the potato with half the sifted flour and carob and half the milk. Stir in the remaining flour, carob and milk. Spread the mixture into the prepared tin. Bake for about 40 minutes. Stand ten minutes before turning on to wire rack to cool. Spread the cold cake with icing.
Carob Icing: Beat the butter in a small bowl until creamy. Gradually beat in sifted icing sugar and carob and milk, beat until icing is spreadable.
