Mix the instant coffee with 2 tbsp boiling water and leave to cool.
Sieve the flour, salt and baking powder into a large, wide bowl. Add the butter and rub in lightly, using your thumb and fingertips, until the mix resembles coarse crumbs. Stir in the sugar and walnuts. Make a well in the centre.
Whisk the eggs and cooled coffee with the milk, add to the dry ingredients and mix to a soft dough. Turn out onto a floured board. Lightly shape into a round. Roll out to about 2.5cm thick and stamp into scones with a 7cm cutter, or cut into wedges. Put onto a baking sheet and bake for 10-15 minutes until golden brown on top. Cool on a wire rack.
When the scones are cool, make the coffee icing. Mix the coffee with 2 tbsp boiling water. Sieve the icing sugar into a bowl, add the coffee and stir in enough boiling water to make a mix with the consistency of thick cream. Spread generously on the scones and sprinkle with a few more nuts. Allow to set, then serve.