Preheat the oven to 180C/350F/gas 4.
Place the egg whites in a large mixing bowl and whisk with the electric hand whisk to soft peaks. Continue to whisk as you add the caster sugar gradually. The mixture should be silky but not too stiff.
Place the egg yolks and caster sugar in the other large mixing bowl and beat with the electric hand whisk until thick and pale in colour.
Sift in the cocoa powder into the mixture, followed by the vanilla extract and fold in.
Add a couple of large spoonsful of the egg white mixture to the cake batter and beat in to loosen it.
Carefully fold in the rest of the egg whites in batches making sure you keep as much air in as possible.
Line the Swiss roll tin with baking paper, leaving a generous overhang at the ends and sides. Stick the corners down with a little butter if necessary.
Pour the cake batter into the tin and bake in the oven for 20 minutes.
Remove from the oven and allow to cool a little before turning the cake out onto another piece of baking paper. Cover loosely with the clean tea cloth.
To make the icing, melt the chocolate in the top of a double boiler or heatproof bowl suspended over a pan of simmering water. Remove from the heat and set aside to cool.
Place 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.
Trim the edges of the Swiss roll and 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. Use the baking paper to help you roll the cake without touching the delicate sponge.
Cut off one end of the ‘log’ (about 5-6cm). Cut this piece diagonally to make branches.
Transfer the cake to a serving plate. Attach the diagonal side of each branch to the cake, one at the top of one side and the other lower down on the other side.
Generously spread the yule log and its branches with the remaining icing.
Create a wood-like texture by marking along the length of the log with a skewer or fork, remembering to diverge occasionally on the main cake to make tree rings.
I usually make some chocolate soil by blitzing pieces of chocolate in the processor and arranging it around the cake. Top this with little branches of holly or ivy and dust the whole cake and it’s surroundings with icing sugar snow.
