Whisk the egg yolks, half the sugar and the vanilla extract together until pale yellow and fluffy. Stir in the mascarpone until smooth and creamy.
Beat the egg whites with the remaining sugar in a separate bowl with a pinch of salt until shiny and stiff.
Fold the egg whites into the mascarpone mixture.
Pour the coffee and liqueur into a dish and quickly dip the biscuits in it, one by one.
Arrange a layer of 12 of the moist biscuits in the base of a large rectangular serving dish. Spoon over half of the mascarpone mixture.
Top with the remaining biscuits and finish with the rest of the mascarpone mixture. Cover and chill in the refrigerator until you’re ready to serve. Just before serving, dust with the cacao.