Peel the oranges and leave them to freeze. Put sugar, water and the grated frozen oranges in a bowl. Cook until the sugar melts, add cocoa, cook for another two minutes, stirring occasionally. Add chocolate and margarine to melt, remove from heat and add ground biscuits. Mix well and divide the mass into two parts. While the mass is still warm, fill the crusts. Load something heavy and let it cool and harden. When the cake has cooled, cover it with chocolate glaze.