1.Preheat the oven to moderate, 180°C/160°C fan-forced. Grease a deep 19cm-square cake pan; line the base and sides with baking paper.
2.Blend the cocoa with the hot water in a large bowl until smooth. Stir in the chocolate, butter, sugar, hazelnut meal and eggs yolks.
3.Beat the egg whites in a small bowl with an electric mixer until soft peaks form, then fold into the chocolate mixture in two batches.
4.Pour the mixture into the prepared pan; bake in a moderate oven for about 1 hour or until firm. Stand the cake for 15 minutes; turn on to a wire rack, top-side up, to cool. Dust with sifted extra cocoa, to serve.
Suitable to freeze. Chocolate suitable to microwave.Note