Preheat the oven to 325°F. Grease a 9x11in baking dish, line with parchment paper and set aside.
Place the softened butter, flour, superfine sugar, and coffee essence in a bowl. Break the eggs in and then beat everything together until smooth and well blended.
Spread the mixture into the prepared baking dish, leveling it smooth with the back of a spoon.
Bake for 25-30 minutes until golden and cooked through. Remove the cake from the baking dish and leave to cool completely on a wire rack.
For the icing, mix the butter, powdered sugar and coffee essence together in a bowl until smooth and well blended.
Spread the icing evenly over the top of the cake. Arrange the walnut pieces all over the top. Cut into wedges and serve.