Preheat the oven to 200°C/Gas 6. Line a rectangular baking tray (about 30 × 40 cm) with greaseproof paper. Sieve the flour and cocoa powder together. Beat the egg whites with the salt and sugar, until they attain a meringue consistency. Mix in the egg yolks, then fold in the flour and cocoa slowly to keep as much air in the mix as possible. Spoon into the prepared tray and bake for 12 minutes or until just cooked through (the tip of a knife inserted into it will come out dry). It must not be overcooked or it will become too dry.
When the cake is cool, turn out of the tray and remove the lining paper. Spread the ganache over it, followed by the chosen filling. Roll it up like a Swiss roll and dust with cocoa powder and icing sugar.