Preheat the oven to 180C/350F/gas mark 4
Use an 18cm/7 inch cake tin.
Grease and line the cake tin. Cream together the butter and sugar until pale and fluffy. Add the sour cream, vanilla sugar, orange peel and almonds and mix well. Blend the carob and boiling water and stir into the mixture.
Sift together the baking powder, salt, potato flour and self-raising flour. Stir the dry ingredients into the mixture alternately with the buttermilk.
Turn the mixture into the prepared cake tin and bake for 1 hour or until a warm knife inserted into the centre comes out clean.
