Again, dissolve your coffee powder in your boiling water and allow to cool until at room temperature.
Sift your icing (confectioners') sugar and set aside.
Give your softened butter a beat for about 10-15 seconds in your mixer, this will ensure your buttercream is soft and creamy.
Leaving your mixer running on a low setting, gradually add your icing sugar a large spoonful at a time, letting each one beat in before adding another.
Once all of your icing sugar has been mixed in, add your cooled coffee and beat until just mixed in, then scrape down the bowl with your spatula so that all of the mixture from the sides and base are mixed in.
Beat one more time for a few seconds to ensure fully mixed.
If using your piping bag, cut the corner off the bag and fit your round nozzle into it, ensuring the bag fits snuggly around it, so that the buttercream does not seep out. Then place in a large glass, fold over the sides and fill the bag with buttercream, using your small palette knife or butter knife.
Roll the sides of the bag back up and tie off the top of the piping bag, or fold it over, so that buttercream does not seep out of the top as you ice.
Starting at one of the shorter end of the top of the cake, begin icing by squeezing the buttercream in a line and then turning and piping another line in the opposite direction, so that an S pattern is formed (see picture in main blog post).
Then take your small palette knife or butter knife and spread the buttercream out across the top of the cake, leaving the edges so that the semi-circle of icing remains (see blog post for picture).
Sprinkle over your chopped walnuts and then place halves down the centre of the cake.