Christmas chocolate log

Method

1. To make the log, whisk the egg yolks with the caster sugar until the mixture is very light and frothy, and leaves a trail when the whisk is lifted out of it. Fold in the cocoa powder.
2. Whisk the egg whites until they are stiff, then fold them into the chocolate mixture.
3. Pour the mixture into the lined Swiss roll tin and spread evenly out to the edges, taking care not to knock out too much of the air.
4. Bake the cake in the centre of a preheated oven at 180°C (350ºF, gas mark 4) for 20-25 mins, or until the cake springs back when lightly touched in the centre.
5. Remove the cake from the oven and turn it out on to a sheet of baking parchment, dusted with icing sugar. Leave the lining paper on the cake, and cover the whole thing with a clean, damp tea towel. Leave the cake to cool completely.
6. To make the filling, bring the cream to the boil and then pour it over the chocolate. Stir until the chocolate melts. Add vanilla extract to taste. Leave the mixture to cool, then chill it in the refrigerator.
7. Whisk the chilled filling until it's light and fluffy.
8. Remove the lining paper from the cake and trim the edges. Spread the filling over the cake and then roll it up with the help of the baking parchment. Dust the top of the cake with icing sugar and cocoa, then transfer it to a serving plate.