Grind the biscuits to make a powder. Mix it with the melted butter and line a greased tart tin to form the Base. Press with fingers to get the shape.
Meanwhile to make the filling, heat the milk and chocolate together on a low flame in a saucepan till the chocolate melts.
Now add the butter, sugar and the cocoa powder into the mix while continuously stirring it.
Make sure that there are no lumps in it. Once everything comes together, add the vanilla extract , and the cornflour dissolved in very little water.
Stir it while the mixture becomes thick enough that it coats the back of the spoon.
Switch off the flame and pour this mixture into a bowl, cover the custard with a cling film making sure that the surface is touching the film. This does not let the custard form a layer.
Once the mixture is cold enough, pour this into the chilled tart. Let it refrigerate and set for atleast 3-4 hours. Then demould it and cut to serve.