Dissolve the coffee in the measurement water. Beat together the butter, sugar, eggs, flour and baking powder in a bowl until pale and creamy. Beat in the coffee and chopped walnuts.
Divide the mixture evenly between 2 greased, baselined and floured 18 cm (7 inch) sandwich tins (see
Make the buttercream. Dissolve the coffee in the measurement water. Beat together the butter, icing sugar and coffee in a bowl until smooth and creamy. Use half the buttercream to sandwich the cakes together, then spread the remaining buttercream over the top and decorate with the walnut halves.