Preheat the oven to 180C/160 Fan/Gas 4. Lightly grease a 33cm x 23cm/13in x 9in Swiss roll tin then line the base and sides of the tin with a large sheet of greaseproof paper, pushing it into the corners. Make a small diagonal snip in each corner of the paper; this helps to fit the paper snugly into the corners of the tin.
Melt the chocolate in a bowl set over a pan of barely simmering water. (Do not let the base of the bowl touch the water.)
Place the egg whites in a large bowl and whisk until stiff but not dry. If you turn the bowl upside down, the whites should be stiff enough not to fall out.
Put the egg yolks in a separate bowl with the sugar and whisk using the same whisk (no need to wash it) on high speed for 2-3 minutes or until thick and creamy and the mixture leaves a thick ribbon-like trail when the beaters are lifted. Pour in the cooled chocolate and gently fold together until well combined.
Gently stir two large spoonfuls of the egg whites into the chocolate mixture to loosen the mix, then fold in the remaining egg whites using a large metal spoon (you don’t want to squash out the air you have just beaten in). Sift the cocoa over the top and lightly fold it in. Pour the mixture into the prepared tin and gently move the tin around until the mixture is level.
Bake in the preheated oven for about 20-25 minutes until risen and the top feels firm and slightly crisp. Remove from the oven, leave in the tin (expect the roulade to fall and crack a little) and set aside until cold.
Whip the cream until it just holds its shape. Lay a large piece of greaseproof paper on the work surface and dust it lightly with icing sugar. Turn the roulade out on to the paper so its lining paper is on top, then carefully peel off the paper. Spread the roulade with the whipped cream, leaving a border of about 2cm/¾in all the way around the edges. With one of the shortest edges facing you, make a cut along it with a sharp knife, going about half way through the sponge. This will help to start the rolling up. Now roll this cut edge over tightly to start with and use the paper to help continue the tight rolling, by pulling it away from you as you roll. Don’t worry if the roulade cracks - that is quite normal and all part of its charm.
Finish with the join underneath then lift the roulade onto a serving plate or board using a large wide spatula or two fish slices. Dust with icing sugar.