Take sugar, cocoa, cornflour in a saucepan, 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 the biscuit and dip it in milk, arrange it in a foil lined plate. Spread the cocoa icing over it and repeat this until the last biscuit. Don't spread icing on top of that biscuit. Put this plate in the fridge for 30 mins. Now take chocolate and butter in a saucepan and melt it on low heat till it is all melted. Pour this over the biscuit and spread on the sides as well. Chill this for 30 mins.