Preheat the oven to 170°C. Lightly grease the Swiss roll tin and line with baking paper, pushing the paper firmly into the corners.
Melt the chocolate in a heatproof bowl set over a pan of gently simmering water, making sure the bottom of the bowl is not in contact with the water. Remove from the heat and set aside to cool slightly.
Beat the egg yolks with the sugar until very pale, thick and creamy. Stir in the slightly cooled chocolate until well blended.
Whisk the egg whites in a large bowl until just starting to form stiff peaks. Stir a large spoonful of egg white into the chocolate mixture to loosen it, then fold in the remaining egg white until evenly blended.
Pour the mixture into the prepared Swiss roll tin and gently level the surface. Bake in the preheated oven for 15-18 minutes, until the top is firm to the touch.
Remove from the oven. Gently slide the cake with the baking paper onto a wire rack and immediately cover with a damp tea towel. Leave to cool completely. If the tea towel dries out, re-dampen it.
When the cake is cool, remove the baking paper. Dust a new sheet of baking paper with icing sugar and invert the cake into it.
Whip the cream until it forms soft peaks and spread it evenly over the top of the cake, leaving about a 2-cm border along one of the long edges.
Roll up the sponge, starting with the long cream –free boarder edge furthest away from you, carefully lifting and peeling off the baking paper as you go. The roulade should have a slightly cracked appearance.
Chill in the fridge for at least an hour before serving to allow it to settle. This will also make the roulade easier to slice. Dust well with icing sugar before serving.