Heat the milk. Mix in a bowl the sugar, the corn flour, the all purpose flour and the egg. Add some of the warm milk and stir with the wire whisk.
Pour the mixture into the boiling milk and keep stirring with the wire whisk until the cream thickens. Remove from heat and let it cool.
Add the grated chocolate and stir the cream again. "Sandwich" two or three biscuits together with spoonfuls of cooled chocolate cream and serve.