Preheat the oven to 180°C/gas mark 4. Grease 2 x 20cm round cake tins and line with baking parchment. Dissolve the coffee granules in 1 tbsp boiling water and allow to cool for 10 minutes.
Meanwhile, beat together the butter and sugar until light and fluffy, then add the eggs, one at a time, beating well after each addition. Using a large spoon, fold in the flour and baking powder. Stir in half the walnuts and the dissolved coffee.
Divide the mixture between the tins and bake for 20-25 minutes, until golden and springy to the touch. Remove the cakes from the tins and leave to cool for 10 minutes on a wire rack.
Dissolve the coffee granules in 1 tsp boiling water. Beat together the butter and icing sugar until pale and fluffy. Stir in the coffee and mix until well combined. Use half the buttercream icing to sandwich the cakes together. Cover the top with the remaining icing and decorate with the remaining walnuts.