Melt the chocolate in a bain-marie (water bath) until it reaches 45°C (113°F). Whisk the egg yolks in an electric mixer fitted with the whisk attachment. Meanwhile, put the sugar and water in a saucepan and bring to the boil. Cook until it reaches 121°C (250°F) and then slowly pour the sugar syrup over the egg yolks and continue to whisk to a full sabayon (until the mix