Crush the biscuits into fine crumbs. Mix with the melted butter and divide half of the mixture between 4 glasses. Next prepare the custard, mix the corn flour and sifted cocoa together and dilute with enough cold milk. Bring the rest of the milk and sugar to almost boiling point. Whisk in the diluted corn flour-cocoa mixture and cook over moderate fire until thickens. To assemble; add a layer of the choc-hazelnut spread over the biscuit crumbs and sprinkle with hazelnuts, spoon over the chocolate custard. Repeat the process again. Chill in a refrigerator and garnish with whipped cream.