Whisk the egg yolks and add sugar, beating well until thick and pale. Then stir the mascarpone into the egg mixture. Whip the cream using an electric mixer, and add to the mixture, mixing well to combine.
Whisk the egg whites in a medium bowl until stiff. Fold 1/3 of the whites into the egg and cream mixture and then fold in the remainder.
Dip the biscuits, one at a time, in the coffee. Place in a square dish, laying them one by one. Spread mascarpone mixture (4-5 tbsp), covering all the biscuits and dust with cocoa.
Repeat layers, spreading more mascarpone mixture into the last layer. Sprinkle cocoa and grated chocolate over the top. Cover tightly and put in the fridge to chill for 5-6 hours. To serve, dust generously with cocoa.
