Blend chocolate in a food processor until it is quite fine. (if you dont have a food processor, you can grate the chocolate instead).
Heat sugar and water over a medium heat, stirring until sugar dissolves. Increase heat and boil for 2 minutes until its slightly thickened.
Pour hot syrup into processor with motor running. Add egg yolks, whole egg and cream. Process until smooth (or beat together in a bowl).
Grease a 22cm cake tin and line with plastic wrap. Leave a border of wrap above the edge of the tin, to make it easy to pull it out of the tin. Pour mixture into tin and freeze for at least 4 hours. Remove from freezer 20 minutes prior to serving.
Pull gently on plastic wrap to release cake from tin.
Dust with cocoa powder, serve immediately with cream, and berries if liked.