Preheat oven to 150 C Grease and line a 20cm spring form cake tin. Place chocolate in a food processor and process until coarsley ground, mix with almonds and set aside. Cream butter and sugar together in an electric mixer until pale and light. Add the egg yolks one at a time and then the ground nuts and chocolate. Beat the egg whites separately until soft peaks form. Fold about 1/4 of egg whites into the chocolate mixture to loosen and then fold in the remaining egg whites. Pour into prepared tin and bake for 45 minutes until set, test by inserting a skewer.