Separate the eggs. Add the yolks to the sugar and beat thoroughly until pale in colour.
When the chocolate has melted, remove it from the heat and stir in the hot water until it forms a smooth mix. Then beat it into the egg yolks and sugar.
Whisk the egg whites until stiff, then fold gently into the chocolate mix.
Pour into a flat tin lined with greaseproof paper. Put in the middle of the oven at 180C for 20-30 minutes.
Take out and allow to cool, still in the tin. When cool, cover with a clean tea towel and leave overnight in the fridge.
Lift out of the tin using the greaseproof paper, put the tin over the top and turn upside down. Remove greaseproof paper.
Cover with whipped cream, and roll like a swiss roll