Preheat the oven to gas mark 5 or 170 degrees fan. In a large bowl, mix together the butter and sugar.
Add the eggs, self-raising flour, baking powder, coffee and walnuts and stir until well combined.
Transfer the cake mixture to a lined loaf tin, and smooth the top evenly with a spatula.
Bake for 40 minutes and leave to cool.
To make the icing, mix together the icing sugar, butter and coffee until it forms a smooth, spreadable icing. Using a palette knife, spread this on the surface of the loaf cake and decorate with walnuts.