1. Finely chop dark chocolate and set aside.
2. In a medium saucepan combine sugar, cornstarch, cocoa powder and salt. Gradually add milk, whisking, until incorporated and smooth.
3. Place the saucepan over medium heat. Cook, stirring constantly until thickens. Add heavy cream, chopped chocolate and whisk until melted and combined. Turn the heat off and add vanilla extract.
4. Transfer to serving dishes. Let cool to room temperature, then refrigerate.
5. Can be served warm or cold.