Preheat the oven to 350°F or 180°C (170°C for a fan oven).
Stir the instant coffee and boiled water together in a small bowl. Set aside to cool a bit more. (You can use 2 tablespoons of perked coffee or espresso in place of the instant coffee and boiling water.)
Grease a 2 pound (23cm x 12cm x 7cm) loaf pan with a bit of the extra butter.
Beat the butter, brown sugar and caster sugar in an electric mixer until thick and pale.
Add the eggs, one at a time. Beat well after each addition.
Mix the gluten free flour and the baking powder together in a small bowl.
With the mixer on low speed, use a large spoon to add the flour and baking powder to the mixture in the bowl of the electric mixer a bit at at time. When all the flour is added, blend throughly.
Stir the millk into the instant coffee and add it to the batter, combining thoroughly.
Add 1 cup (120 grams) of the chopped walnuts and blend into the batter. (Set aside the rest of the walnuts to use for decoration.)
Transfer the batter to the loaf pan and bake for 35 to 45 mnutes, or until a skewer inserted in the centre of the cake comes out clean. It may take a little more time, depending on your oven.
Remove the Gluten Free Coffee and Walnut Cake from the oven and let it cool in the pan for 15 to 20 minutes, and then turn it out on a wire rack to cool completely.
When the cake is completely cool, make the glaze. Put the icing sugar in a medium bowl. Add the water, a little at a time, blending thoroughly after each addition, until a drizzle-able consistency is reached. You may not need all the water, or you may need a tiny bit more.
Drizzle about half the glaze over the cake. Set the remaining glaze aside.
Sprinkle the remaining chopped walnuts over the top. If you have time, let the glaze and the walnuts set for about 30 minutes. However if time is short you can continue on with the next step.
Drizzle the remaining glaze over the top. (Don't worry if you have some left over.) Let the glaze set and then serve.
If you do not need the recipe to be gluten free, substitute 1⅓ cups (200 grams) all-purpose (plain) flour for the 1¼ cups gluten free flour blend and reduce the milk from ¼ cup to 2 tablespoons (30 ml).