Preheat oven to 180C. Grease and line a 30cm square pan with baking paper.
Combine chocolate, butter, cocoa and oil in a bowl over a pan of simmering water (make sure the bowl doesn’t touch the water), stirring until melted. Cool slightly.
Meanwhile, beat eggs, vanilla, coffee and sugar in an electric mixer on medium speed for 10 minutes or until thick and pale. Fold warm chocolate mixture into the egg mixture until combined, then fold in flour and walnuts until just combined.
Pour mixture into the prepared pan and bake for 40 minutes or until a toothpick inserted in the middle comes out with a few moist crumbs, but is not sticky.
Cool completely in pan before turning out and cutting into 16 squares. Dust with icing sugar and serve.