Prepare the Biscuit Mixture: In a large mixing bowl, combine the crushed biscuits with cocoa powder and a pinch of vanilla. Mix these dry ingredients until well combined.

Cook the Sugar Syrup: In a saucepan over medium heat, combine the sugar and milk. Stir continuously until the sugar is completely dissolved and the mixture is just about to boil. Remove from heat and add the melted butter, mixing well until the butter is fully incorporated into the syrup.

Combine and Set: Pour the warm sugar syrup over the biscuit and cocoa mixture. Stir everything together until the biscuits are evenly coated with the chocolate syrup.

Chill: Place the cake in the refrigerator to set for at least 1 hour, or until firm.

Serve and Enjoy: Once set, remove the cake from the mold. Slice and serve as is, or with a dollop of whipped cream or a sprinkle of powdered sugar for an extra touch of sweetness.