Preheat the oven to 180 degrees Celsius Grease and line a 30cm square pan with silicone paper.
Combine the chocolate, butter, cocoa powder and oil in a bowl over a pan of simmering water, stirring. Cool slightly.
Beat the eggs, vanilla, coffee and sugar with an electric mixer on medium speed for about 10 minutes or until thick and pale. Fold the warm chocolate mixture into the egg mixture until combined, then fold in the flour and walnuts until just combined.
Pour the mixture into the pan and bake for about 4o minutes or until a skewer inserted into the centre comes out with a few moist crumbs, but is not sticky.
Cool completely in the pan before turning out and cutting into 16 squares-dust with icing sugar before serving.