Start by making the sponge.
In a bowl, add the butter and sugar and cream until smooth.
Add the eggs one by one, mixing well to incorporate.
Add the sifted flour, instant coffee, baking powder and chopped walnuts, and mix well.
Preheat the oven to 180 degrees Celsius (350 Fahrenheit).
Grease and flour 2 round cake tins with detachable walls (mine have a diameter of 20 cm /8 inches).
Pour the batter in, and level it with the back of a spoon or a spatula.
Bake for 20-25 minutes or until a skewer inserted in the middle of the cakes comes out clean.
Transfer to a cooling rack and leave to cool completely.
To make the icing, sift the icing sugar, add the soften butter, expresso coffee and milk to a food processor, and blitz until smooth.
Spread half of the icing over the first sponge, top with the second sponge, and ice the top with the remaining buttercream.
Decorate with walnut halves.