1. Prepare a Swiss roll flat tin or similar, 23 x 30cm, by lining it with baking parchment. Preheat the oven to 180C/350F/gas 4.
2. First melt the chocolate – break it into little pieces and melt over bain-marie or in a bowl over a pan with simmering water (don’t let the bowl touch the water). Leave to cool.
3. Put the egg yolks with the 35g caster sugar in a bowl (or in a standing mixer bowl) and beat until pale and fluffy and ribbons form over the surface when the beaters are lifted. Add the vanilla extract and beat in. Add the melted chocolate and beat until combined.
4. Change the mixer attachment to a whisk (balloon on the standing mixer) and beat the egg whites until foamy, add the cream of tartar and beat until soft peaks form. Gradually add the 20g of sugar and beat until the mixture forms stiff peaks (a good trick is to turn the bowl upside down – if the egg whites stay in, they’re done. If they fall out, you have to start over again…).
5. Fold a small amount of the egg whites into the egg yolk mix with a spatula, to loosen it up. Then add the rest of the egg whites and fold carefully not to deflate it, small streaks of white are acceptable. Pour it into the prepared tin smoothing the surface with a spatula.
6. Bake for 15-17 minutes – when you press the surface with your finger, it should spring back. It will have puffed up considerably.
7. Place the tin on a wire rack and cover with a damp towel – I find it’s best not to let the towel touch the cake, so I build a construction of upturned cups, another wire rack suspended on top of them and the damp tea towel hanging over the cake.
8. When the cake is cooling, prepare the filling: stir the sugar and the cocoa into the cream, stick the hand mixer beaters in and chill in the fridge for an hour – by that time the cake will have cooled completely.
9. Beat the cream until soft peaks form, you don’t want to overbeat it, stop when it looks like whipped cream. Spread the cream over the cooled cake still on the parchment, reserving a little bit, then roll up along the longer side using the paper to help you – peel it off gradually as you roll. It can crack – no matter, it will look more authentic, like a log.
10. Place it on a tray seam side down. Cut off a quarter at an angle and stick it on at a side, like an end of a branch, using the reserved cream. Dust generously with icing sugar just before serving.