Preheat the oven to 350 degrees F / 180 C. Line a loaf pan with parchment paper.
Mix the coffee granules with the boiling hot water until the coffee dissolves, set aside to cool.
Sift the flour, baking powder, cinnamon and salt.
Using a stand mixer beat the softened butter and brown sugar until smooth, light and fluffy.
Add the vanilla extract and one egg at a time, beating until the egg is well incorporated before adding another egg.
Combine the cooled coffee and flour with the butter mixture, beat until just combined.
Fold in the chopped walnuts.
Transfer the cake batter into the prepared loaf tin and bake for 40-50 minutes or until an inserted skewer comes out clean. Cover the cake with a sheet of foil after 15 minutes if the top of the cake is beginning to brown quickly.
Cool for 5 minutes before removing from the tin and cooling completely on a cooling rack.
When the cake is completely cooled prepare the frosting by mixing the coffee granules with boiling water and set aside to cool.
Beat the butter in a stand mixer until light and fluffy.
Slowly add half of the powdered sugar, beating in between each addition.
Combine the coffee and beat until smooth.
Spread the frosting over the cooled cake and decorate with chopped walnuts.
Place the cake in the fridge to chill for an hour before slicing and serving.