Ingredients 

for the cake 

• 6 large eggs (separated)
• 150 grams caster sugar
• 50 grams cocoa powder
• 1 teaspoon vanilla extract
• 5 teaspoons icing sugar (to decorate)

 for the icing 

• 175 grams dark chocolate (chopped)
• 250 grams icing sugar
• 225 grams soft butter
• 1 tablespoon vanilla extract

 Method 

1. Preheat the oven to 180°C/gas mark 4/350°F.
2. In a large, clean bowl whisk the egg whites until thick and peaking, then, still whisking, sprinkle in 50g/1/4 cup of the caster sugar and continue whisking until the whites are holding their peaks but not dry.
3. 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.
4. Lighten the yolk mixture with a couple of dollops of the egg whites, folding them in robustly. Then add the remaining whites in thirds, folding them in carefully to avoid losing the air.
5. 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.
6. 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.
7. To make the icing, melt the chocolate – either in a heatproof bowl suspended over a pan of simmering water or, my preference, in a microwave following the manufacturer’s guidelines – and let it cool.
8. 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. You can do this by hand, but it does mean you will have to sieve the sugar before creaming it with the butter and stirring in the chocolate and vanilla.
9. 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.
10. Cut one or both ends slightly at a gentle angle, reserving the remnants, and place the Swiss roll on a board or long dish. The remnants, along with the trimmed-off bits earlier, are to make a branch or two; you get the effect by placing a piece of cake at an angle to look like a branch coming off the big log.
11. Use remaining icing to decorate the log. A fork can be used to create a wood like effect.