Melt chocolate in a heatproof bowl over a saucepan of simmering water.
Meanwhile, combine sugar and 40ml water in a small saucepan, stir over medium-high heat until sugar dissolves, then bring to the boil. Cook until mixture reaches 115C on a sugar thermometer (1 minute).
Meanwhile, whisk yolks and egg in an electric mixer until pale. Whisking continuously, add hot syrup in a thin steady stream down side of bowl, then whisk until cooled to room temperature (15-20 minutes). Fold through chocolate and set aside.
Whisk cream, crème fraîche and liqueur in a bowl to soft peaks, fold into chocolate mixture and refrigerate until chilled and set (30-40 minutes).