Stir the crushed biscuits with the cocoa and melted butter. Distribute the mixture along the bottom and walls of the form and put it in the fridge.
Stir until you obtain a smooth mixture. Pour the 2 creams over the cake layer simultaneously, alternating between them in the inner circles to obtain the "zebra" effect. Bake the cake at 338°F (170 °C) for 50 min. Take it out and leave to cool for 6-7 hours in the fridge.