Preheat oven to 160°C and grease and line a 8 x 4 inch loaf pan with baking paper.
In a medium bowl add flour, baking powder, and cinnamon. Use a whisk to combine until evenly mixed through. Set aside.
In a large bowl, add oil and sugar and whisk together. Add in eggs and continue whisking until all combined. Slowly add in dry ingredients in three batches, alternating with the coffee and milk, mixing in between each until the batter comes together. It will be quite thick. Fold through walnuts with a spatula and pour mixture into the lined and greased pan.
Bake for 50-60 minutes, or until a skewer when inserted comes out clean. Allow to cool in the pan for 10 minutes before turning out onto a wire rack to cool completely.
To make the coffee icing. Add the butter and icing sugar to a bowl and begin to beat with an electric handheld mixer or stand mixer with the paddle attachment fitted. Slowly add the coffee ½ a tablespoon at a time until your desired consistency is reached, being careful not to make it too thin. If that happens, just add more icing sugar to thicken.
Spread icing over the top of the cooled cake and top with more crushed walnuts if desired. Slice and serve.
Cooking time may vary based on the size of your loaf pan and even your individual oven. Cake is cooked when a skewer when inserted in the centre comes out clean. If not done after 50 minutes, check in 5 minute increments until cooked.I use Nuttelex Buttery to replace butter in most of my recipes as it is dairy-free and gives a 'buttery' flavour without being greasy. As this is an Australian product, the closest US alternative I believe is Earth Balance. Otherwise any dairy-free spread will work.