Preheat the oven to 180°C/160°C Fan/gas mark 4/350ºF.
In a large, clean bowl whisk the egg whites until thick and peaking, then, still whisking, sprinkle in 50g / ¼ cup of the caster sugar and continue whisking until the whites are holding their peaks but not dry.
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.
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.
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.
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.