Hold the sieve over the bowl of eggy chocolate mixture and resift the cocoa and flour mixture, shaking the sieve from side to side, to cover the top evenly.
Pour the cooled chocolate mixture over the eggy mousse, then gently fold together with a rubber spatula. Plunge the spatula in at one side, take it underneath and bring it up the opposite side and in again at the middle. Continue going under and over in a figure of eight, moving the bowl round after each folding so you can get at it from all sides, until the two mixtures are one and the colour is a mottled dark brown. The idea is to marry them without knocking out the air, so be as gentle and slow as you like.