1. Preheat oven to 180°C conventional or 160°C fan-forced. Grease and line a 22cm square cake tin with baking paper, allowing paper to overhang the edges of the pan.
2. Melt chocolate, butter, coffee and water in a saucepan over low heat until smooth.
3. Combine sifted flour, cocoa, sugar and walnuts in a large mixing bowl. Add chocolate mixture and eggs, then stir well. Pour mixture into prepared tin.
4. Bake for 40 minutes. Allow to cool in tin for 15 minutes before using the overhanging baking paper to lift out onto a wire rack to cool. Once cool, cut into 20 slices and dust with pure icing sugar.