Preheat the oven to 160/Gas 3. Chop the walnuts in a food processor until the consistency of fine breadcrumbs. Melt the chocolate over a bowl of simmering water. Separate the eggs. Beat the egg yolks until they are pale in colour. Add the melted chocolate into the beaten eggs yolks and then fold the ground walnuts into the mix. Beat the egg whites until they have stiff peaks and fold them into the chocolate mixture. Pour into a 25cm cake tin lined with parchment paper. Bake for ten minutes then reduce the heat to 150/Gas 2 for a further 45 minutes. The cake will be crumbly and moist with a delicious rich chocolate taste.