Chop the chocolate and couverture. Melt coconut oil, chocolate and couverture over a hot water bath.
Cream eggs, icing sugar, cocoa and rum with the whisk of the hand mixer. Stir in the coconut fat chocolate mixture well. Place back on the hot water bath.
A box mould (23 cm long; 1.5 litre capacity); line with foil. Spread some chocolate cream on the bottom. Place 4 biscuits next to each other on top. Repeat until all the biscuits and chocolate cream are used up. Finish with the biscuits.
Leave to set in the refrigerator for at least 6-8 hours (it is best to prepare the cake the evening before). Turn the cake over onto a plate. Remove the foil. Dust the cake with cocoa.