Roughly chop both the dark and whole milk couvertures. Cut the coconut fat into small pieces.
In a saucepan over medium heat, melt the chopped couvertures and coconut fat together with the whipped cream, stirring constantly until the mixture is smooth.
Remove from heat and stir in the vanilla sugar.
Place a layer of biscuits in the bottom of the prepared loaf pan (the biscuits may break).
Spread a layer of chocolate cream over the biscuits, ensuring they are completely covered.
Continue alternately layering chocolate cream and biscuits in the loaf pan until you have 7-8 layers total.
Cover the loaf pan and refrigerate for about 5 hours, or preferably overnight, until the cream is set.
Run a knife around the inside edges of the loaf pan to loosen the pastry. Invert the loaf pan onto a plate to turn the pastry out, with the freezer bag still attached.
Carefully peel the freezer bag away from the pastry.