Beat the egg yolks and sugar together in a medium bowl until thick and pale. Add the mascarpone and stir to combine.
Using an electric mixer, whip the egg whites until soft peaks form. Stir one third of the whites into the mascarpone mixture, then gently fold in the remainder.
Pour-in the coffee in a small bowl. Dip half the biscuits, one at a time, in the coffee to moisten. Arrange in a single layer in a deep 20-cm (8-in) square dish.
Spread mascarpone mixture and dust with cocoa or chocolate crushed finely. Repeat layers. Cover more cocoa or chocolate the last layer.
Cover with cling wrap and refrigerate for at least 4 hours. To serve, we can dust generously with more cocoa, if desired. Serves 8