Preheat the oven to 180°C/160°C Fan/gas mark 4/350°F. In a large, clean bowl whisk the egg whites until thick and peaking, then, still whisking, sprinkle in 50g / ¼ cup of the caster sugar and continue whisking until the whites are holding their peaks but not dry. In another bowl, whisk the egg yolks and the remaining caster sugar until the mixture is moussy, pale and thick. Add the vanilla extract, sieve the cocoa powder over, then fold both in. Finally, gently fold in the egg yolks.
Line a Swiss roll tin with baking parchment, leaving a generous overhang at the ends and sides, and folding the parchment into the corners to help the paper stay anchored. Pour in the cake mixture and bake in the oven for 20 minutes. Let the cake cool a little before turning it out onto another piece of baking parchment. If you dust this piece of parchment with a little icing sugar it may help with preventing sticking, but don’t worry too much as any tears or dents will be covered by icing later. Cover loosely with a clean tea towel.
To make the icing, melt the chocolate in a heatproof bowl suspended over a pan of simmering water, then let it cool. Put the icing sugar into a processor and blitz to remove lumps, add the butter and process until smooth. Add the cooled, melted chocolate and the tablespoon of vanilla extract and pulse again to make a smooth icing.
Sit the flat chocolate cake on a large piece of baking parchment. Trim the edges of the Swiss roll. Spread some of the icing thinly over the sponge, going right out to the edges. Start rolling from the long side facing you, taking care to get a tight roll from the beginning, and roll up to the other side. Pressing against the parchment, rather than the tender cake, makes this easier.
Spread the yule log with the remaining icing, covering the cut-off ends. Create a wood-like texture by marking along the length of the log with a skewer or sharp knife and finish with a handful of cranberries.