Combine cream and milk in a medium saucepan over medium heat and bring to just a simmer.
Meanwhile, whisk yolks, cornflour, vanilla and sugar together in a large heatproof bowl. Gradually whisk in the hot cream mixture until smooth.
Return mixture to cleaned saucepan and place over low heat. Cook, stirring constantly, until custard thickens and thickly coats the back of a spoon. Set aside to cool slightly.
Divide custard evenly between 2 heatproof bowls. Add chopped chocolate to one bowl and stir until well combined and smooth. Add brandy to second bowl and stir until well combined. Cover surface of both bowls with plastic wrap (to prevent a skin from forming). Set aside to cool, then refrigerate until chilled.
Divide chocolate custard evenly between 4 serving glasses, then top with a layer of the brandy custard. Garnish with chocolate curls and serve with biscotti.