Cream together the butter or margarine and sugar. Add the flour, cocoa powder and eggs to the bowl and beat until smooth. Reserve half the pieces of flaky chocolate and 50 g (2 oz) each of the hazelnuts and plain dark chocolate. Fold the remainder into the cake mixture with the raisins.
Grease and line a 20 cm (8 inch) round or 18 cm (7 inch) square cake tin. Turn the mixture into the tin and scatter with the reserved chocolate and nuts. Bake in a preheated oven, 150°C (300°F), Gas Mark 2, for about 2 hours or until a skewer inserted into the centre comes out clean.
Leave to cool in the tin. Serve lightly dusted with cocoa powder or icing sugar.