Whisk egg whites until stiff. Whisk yolks with sugar until thick and creamy. Mix in some of the rum. Mix in the mascarpone. Integrate the egg whites, carefully. Mix coffee with rum and liqueur. Dunk biscuits one by one in liquid and lay in shallow dish to cover the bottom. Spread half the cream on top, sprinkle with chocolate. Lay the remaining biscuits, dunked, on top. Then spread the remaining cream. Sprinkle with chocolate. Cover with cling film and chill for min. 2 hours.