1. Using an electric hand mixer, beat egg yolks and sugar together in a large bowl until pale and creamy. Gently fold in mascarpone and liqueur. Wash and dry beaters.
2. Whip cream to form peaks. Fold into mascarpone mixture. Wash and dry beaters.
3. Beat egg whites until stiff peaks form. Fold into mascarpone mixture. Cover with plastic wrap and refrigerate for 30 minutes.
4. Combine coffee and warm water in a shallow dish, stirring until coffee dissolves. Using half the biscuits, dip biscuits 1 at a time in coffee, turning to coat.
5. Place, in a single layer, on the base of a glass (or if making a whole cake, over the base of a 6cm deep baking dish).
6. Spread enough mascarpone mixture over biscuits to cover. Repeat layers with remaining biscuits, mascarpone mixture and flake.
7. Cover and refrigerate for 4 hours.
8. Dust top with flake or cocoa powder and serve.