Preheat the oven to 180°C.
Grease a Swiss roll tin with butter and line it with greaseproof paper.
Melt the chocolate in a glass bowl over a saucepan of boiling water.
Beat the egg yolks into the sugar.
Pour the cooled, melted chocolate into the egg and sugar mixture.
Whisk the egg whites to stiff peaks and fold them into the chocolate mix.
Pour the batter into the tin and bake for 15-20 minutes until firm.
Wring out a damp cold tea towel.
Cover the roulade with the tea towel for ten minutes, then refrigerate overnight.
Dust a fresh sheet of greaseproof paper with icing sugar.
Turn the cake onto the paper and remove the original lining.
Whip the double cream and spread it over the cake.
Roll up the roulade using the greaseproof paper for support.