Preheat the oven to 180C/350F/gas mark 4
Use a 1.25 litre/2 pint ring mould.
Lightly grease the ring mould. Cream the butter with the sugar until light and fluffy. Gradually beat in the sour cream. Mix the coffee powder and warm milk and beat into the mixture. Fold in the flour and chopped walnuts, then stir in the milk. Spoon into the prepared mould and smooth the surface.
Bake in the preheated oven for 45 - 50 minutes until well risen and firm to the touch. Cool slightly before carefully turning out of the mould onto a wire rack.
Glacé icing: Mix the coffee powder with the hot water. Add to the icing sugar, mixing in well. Carefully pour over the ring, allowing some icing to trickle over the sides. Decorate with the walnut halves.
