Preheat oven at 180C/350F/Gas 4
Line or grease a loaf tin.
Crush the walnuts using a food processor. They should be as fine as possible.
Mix the instant coffee with the boiling water to make a paste.
Put the rest of the ingredients in a large mixing bowl and mix well.
Fold the coffee paste and the walnuts into the mixture.
Pour the cake batter mixture into the prepared loaf tin and bake in the middle of the oven for 50-60 minutes until brown on the top and a skewer comes out clean. Leave to cool completely on a wire rack.
To make the coffee icing, mix the instant coffee with the boiling water.
Add the icing sugar and butter to a large mixing bowl along with the coffee and mix well until you have created a light, rich, coffee icing.
Prepare the cake by levelling the top if needed and cutting in half lengthways.
Pipe half the icing on top of the cake base before putting the top half back in place. Ice the top of the cake with the rest of the coffee icing.
Sprinkle with crushed walnuts and serve.