Preheat the oven to 180C/350F/Gas 4.
Prepare a Swiss roll tin by greasing it with butter and then lining it with a piece of greaseproof paper slightly larger than the tin.
Gently melt the chocolate in a glass bowl set over a saucepan of boiling water.
Beat the egg yolks into the sugar.
When the chocolate has cooled and melted a little, pour it into the egg yolk and sugar mixture.
Beat the egg whites to stiff peaks, before folding into the chocolate mix.
Pour the mix into the tin and bake in the oven for 15-20 minutes until firm to the touch - you will know the cake is ready when an inserted skewer comes out clean.
Run a tea towel under cold water and wring out.
Place the tea towel over the chocolate roulade once it has cooled a little. After ten minutes remove the tea towel, transfer the roulade to the fridge and leave overnight.
The next day, sieve icing sugar over a sheet of greaseproof paper a little larger than the tin.
Tip the cake out onto the fresh greaseproof paper and peel off the other piece of greaseproof paper from the top of the roulade.
Whip the double cream and smooth it all over the roulade.
Roll up the roulade, using the greaseproof paper to help hold it together. It may crack a little, but this will not affect the quality of the cake.