Whisk the egg yolks and sugar in a bowl with a handheld electric whisk until light, airy and the beaters leave a trail when lifted. Put the mascarpone in a large bowl and beat in one-third of the egg mixture until smooth, then fold in the remaining egg mixture. Beat the egg whites in a separate bowl until they hold their shape and have a consistency resembling shaving foam. Fold into the mascarpone mixture.
Pour the coffee and Marsala into a shallow bowl and dip in the sponge fingers until soaked on both sides but not all the way through. Arrange a layer of tightly packed biscuits in four 10 cm (4 inch) bowls. Spread with half the mascarpone mixture, then top with a second layer of soaked sponge fingers. Use the remaining mascarpone mixture to cover the sponge fingers. Cover with clingfilm and chill for at least 3 hours or up to overnight.
Just before serving, remove the tiramisu from the refrigerator and dust with the grated chocolate.