Take sugar, cocoa, cornflour in a sauce pan, add in vanilla and milk. Mix well. Cook this on low heat till it thickens. Now set aside to cool it a bit.
Now take biscuit and dip it in milk, arrange it in a foil lined plate. Spread the cocoa icing over it and repeat this untill the last biscuit.
Dont spread icing on top of that biscuit. Put this plate in fridge for 30 mins.
Now take chocolate and butter in a sauce pan and melt it on low heat till it is all melted. Pour this over the biscuit and spread on the sides as well.