Beat the egg whites and sugar to snow and add them to the yolk and mascarpone mixture. Dip the biscotti in the coffee until well soaked and arrange in a tray.
Smear with the resulting cream on top and arrange a layer of biscotti, layer of cream. After the last layer of cream, sprinkle on the cocoa with a sieve.
Cover the tray with foil and leave it in the fridge for at least 6-7 hours.