Preheat oven to 160 degrees Celsius Line a 27cm spring form pan with foil. Place the chocolate and butter in bowl over a saucepan of simmering water until melted. Remove from the heat and allow to cool.
Place eggs and sugar in a large bowl and beat with an electric beater for five minutes until thickened. Fold the cooled chocolate mixture into egg mixture stirring until well combined and reduced in volume.
Pour into the prepared pan, then place in a large baking tray and pour boiling water into the tray to come halfway up the sides. Bake in the over for one hour. Turn off the oven and leave to cool completely in the oven. Serve thin slices of the cake with a dollop of crème fraiche.