Put butter and brown sugar in a large mixing bowl. Stir together until they are nice and creamy. Stir in melted chocolate.
Sift together in a separate mixing bowl the flour, mace and cinnamon. Stir this in with ingredients in first bowl, mixing with cold coffee. Drop in eggs, one at a time, mixing each in thoroughly. Set aside momentarily.
Grease and lightly flour 2 large loaf tins. Divide cake mixture between tins.
Bake in a preheated oven at 180 degrees C for about 50 minutes. When tests done, let cake cool in tin. Turn cooled cake out on plate.
Put icing sugar into bowl and slowly mix in cold coffee and vanilla. Start out with a little coffee; don't pour all in at once. You may not need the entire 250ml of coffee.
Spread icing over the cakes and sprinkle over walnuts.