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.
