Cut the butter into small pieces. In a double boiler melt the chocolate with the butter, mixing with a wooden spoon until you get a smooth cream. Add the cocoa powder, cognac and the egg yolks and stir.
Break the biscuits into small pieces. Add them to the chocolate mixture and then add the hazelnuts . Mix well and allow it to cool. As soon as it has cooled, spread on greaseproof paper which you have previously sprinkled with powdered sugar. The mixture should be shaped into a cylinder. Wrap it tightly and put in the freezer for 24 hours.