Preheat oven to 180 C / Gas 4. Grease and flour a 20cm square cake tin. Sift together the flour, 5 tablespoons carob powder, bicarbonate of soda and salt. Set aside.
In a large bowl, cream together the 110g butter and 225g of honey until light and fluffy. Beat in the eggs one at a time, then stir in the banana and vanilla. Beat in the flour mixture alternately with the 175ml water. Stir in chopped walnuts. Pour cake mixture into prepared tin.
Bake in the preheated oven for 35 to 40 minutes, or until a skewer inserted into the centre of the cake comes out clean. Allow to cool.
To make the icing: In a large bowl, cream 50g butter and 5 tablespoons honey until smooth. Blend in the milk powder, carob powder, 4 tablespoons water and 1 teaspoon vanilla. Beat until smooth and spread on cooled cake.