Heat oven. Heat the oven to 180°C (160°C fan, gas 4). Grease a 29 x 18cm Swiss roll tin and line the base and sides with baking parchment.
Melt chocolate. Put the chocolate and water in a heatproof bowl over a pan of barely simmering water. Stir until smooth, then take off the heat.
Make mousse. Beat the 2 yolks, then beat them into the warm chocolate. Cool a little. Whisk the 2 whites until stiff and fold them in. Cover and chill for about 1 hour.
Whisk yolks. Whisk the 6 sponge yolks until they start to thicken. Add the caster sugar and whisk until slightly thicker, but not very thick.
Add cocoa. Sift in the cocoa and mix until combined.
Whisk whites. In a clean bowl, whisk the 6 sponge whites to soft peaks.
Fold gently. Stir in a third of the whites to loosen the mixture. Fold in the rest with a metal spoon until no streaks remain.
Bake sponge. Pour into the tin and tilt to level. Bake on the middle shelf for 20 to 25 minutes, until it springs back when pressed.
Cool completely. Leave the sponge in the tin until cold. It will sink.
Turn out. Dust a fresh sheet of parchment with icing sugar. Turn the sponge onto it and peel off the lining paper.
Fill sponge. Spread the chilled mousse over the sponge. Whip the cream softly and spread it on top, leaving a narrow bare edge.
Roll up. Starting at a short end, use the paper to roll the sponge into a log. Dust with icing sugar, chill until serving and cut into 8 slices.
