Chop both chocolate coatings and break coconut oil into pieces. Melt the couverture and coconut oil in a hot water bath. Mix eggs, icing sugar, milk and cocoa with the whisk of the mixer until creamy. Add to the couverture mixture.
Rinse out a box mould (26 cm long, 1.5 l capacity) and line it with foil. Alternate layers of chocolate cream and biscuits until all the biscuits and chocolate cream are used up, finishing with chocolate cream.
Cover with foil and chill for at least 5 hours, preferably overnight.
Cold dog from the mold and remove the foil. Cut into slices.