In a small bowl whisk 1/2 cup milk (120ml) with the cornstrach until no lumps. Set aside.
In a saucepan place the remaining milk, heavy cream, sugar, cocoa powder and heat until sugar is melted. When sugar is melted and the mixture is simmering, reduce to medium-low heat, pour in milk-cornstrach mixture and stir until thickens. Turn the heat off and add chopped chocolate, stir until melted. Set aside.
Dip the biscuits in the milk/coffee and arrange in the bottom of 13"X9" (32X22cm) dish. Spread 1/3 of the chocolate cream over the biscuit layer. Then arrange another layer of dipped biscuits. Spread another 1/3 of cream over the biscuits. Arrange last (third) layer of dipped biscuits on top and spread the last layer of chocolate cream.