Preheat the oven to 180C/350F/gas mark 4
Use a 23cm/9 inch square slab tin.
Grease and line the cake tin. Melt the carob chips with the water in heatproof bowl over hot water, cool to room temperature.
Beat together the ingredients in large bowl. Add the carob to the mixture. Beat vigorously for about 3 minutes or until mixture changes in colour and becomes smooth. Pour into the prepared tin, bake for about 40 minutes. Stand 5 minutes before turning on to wire rack to cool.
Spread the cold cake with the peppermint cream. Refrigerate for one hour. Spread the icing over the cream, then refrigerate until set.
Peppermint Cream: Cream butter until light and fluffy, gradually beat in sifted icing sugar, then milk and essence.
Carob Icing: Sift icing sugar and carob powder into a bowl, stir in the combined butter and water. Beat until smooth.
