1.Preheat oven to 180°C (160°C fan-forced). Grease deep 20cm-round cake pan; line base and side with baking paper.
2.Blend cocoa with the water in large bowl until smooth. Add chocolate, butter, sugar, hazelnut meal and egg yolks; stir until combined.
3.Beat egg whites in small bowl with electric mixer until soft peaks form; fold into chocolate mixture in two batches.
4.Pour mixture into pan; bake about 1 hour. Stand cake in pan 15 minutes before turning, top-side up, onto wire rack to cool. Dust with sifted extra cocoa.