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. Gently fold in this powder using the same figure of eight action as before. The mixture will look dry and dusty at first, and a bit unpromising, but if you keep going very gently and patiently, it will end up looking gungy and fudgy. Stop just before you feel you should, as you don’t want to overdo this mixing. Finally, stir in the white and milk chocolate chunks until they’re dotted throughout. Now your mixing is done and the oven can take over.