Put the egg yolks and sugar in a medium bowl and place it over a pan of simmering water. Use an electric whisk to beat the mixture until it is pale and fluffy. Remove the bowl from the heat and continue to whisk until it is really thick. Set aside to cool slightly.
Mix the coffee and liqueur together in a shallow dish. Working one at a time, dip 8 sponge fingers in the coffee and lay them across the bottom of a rectangular glass dish.
Fold the Quark into the egg-yolk mixture, then spread half of this over the sponge fingers. Mix the grated chocolate and cocoa together and sprinkle a couple of spoonfuls over the wet mix.
Dip the remaining sponge fingers in the coffee liquid, lay on top of the chocolate and cover with the remainder of Quark mixture. Finally top with the last of the grated chocolate mixture. Chill for at least 2 hours before serving.