Steps
- Preheat the oven to 350°F / 180°C. Line a 9 × 13in / 23 × 33cm jelly roll pan with parchment paper.
- Separate 6 large eggs. Whisk the whites until thick and peaking, then whisk in a third of the 150g of superfine sugar until stiff but not dry.
- Whisk the yolks with the remaining sugar until pale, thick, and moussy. Fold in 1 teaspoon of vanilla extract and 50g of sifted cocoa powder.
- Fold in a couple of spoonfuls of the whites to loosen, then the rest in thirds. Pour into the pan and bake for 20 minutes. Cool completely.
- For the icing, melt 175g of chopped dark chocolate and let it cool. Process 250g of icing sugar in a food processor to remove any lumps, then process in 225g of softened butter, the chocolate, and 1 tablespoon of vanilla extract.
- Turn the sponge out onto a sheet of parchment paper and trim the edges. Spread with a thin layer of icing right to the edges, then roll up from a long side, using the paper to help.
- Set the log on a plate, cover it completely with the remaining icing, and dust with icing sugar.