In a mixing bowl, whisk together the sugar, cocoa, cornstarch and salt. In a thin stream, pour in the milk, whisking until smooth. In a thin stream, add in the half and half, and whisk until smooth. Pour the mixture through a fine sieve into a saucepan. Over medium heat, whisking the mixture constantly, bring to a boil. Boil gently 2 minutes.

Turn off the heat, and stir in the bittersweet chocolate and vanilla extract. Transfer the custard to a mixing bowl. Fill a large bowl with ice cubes, rest the mixing bowl on top, and add cold water to cover the ice cubes. Let the mixture cool, stirring frequently with a rubber spatula, about 15 minutes. Fold in the white chocolate and pour the mixture into 1cup ramekins, cups or mugs. Refrigerate until well chilled. Serve cold with dollops of whipped cream.