1.Preheat oven to 160°C. Grease deep 20cm round cake pan and line base and side with baking paper.
2.Combine butter and chocolate in a small saucepan; stir over low heat until smooth. Cool 10 minutes.
3.Beat egg yolks and sugar in medium bowl with electric mixer until thick and pale. Beat in chocolate mixture.
4.Beat egg whites in separate small bowl with electric mixer, until soft peaks form.
5.Fold ground hazelnuts into chocolate mixture. In two batches, fold in egg white. Spoon mixture into pan and bake about 1 1/2 hours.
6.Cool cake in pan. Turn cake top-side down onto serving plate.
7.To make chocolate ganache: bring cream to the boil in small saucepan. Remove from heat, add chocolate; stir until smooth. Stand 5 minutes before using.
8.Spread cake with ganache; top with nuts.