Break or chop the chocolate into smallish pieces, so that it will melt more quickly as the pudding cooks.
Combine the cornstarch, sugar, and salt in a saucepan, and whisk it together. Gradually add about a third of the milk, whisking until smooth, then add in the rest of the milk and cream. Add the chopped chocolate, then put the saucepan over medium heat, and bring it to a boil, stirring constantly. If any lumps form, whisk vigorously!
Boil for exactly one minute. The mixture will thicken considerably. Remove from the heat, and stir in the vanilla.
Divide among 6 custard cups (or teacups or small bowls or whatever.) Some people do not enjoy any ‘skin’ on their pudding. If you dislike it, place plastic wrap right on top of the pudding and smooth it gently against the surface. Otherwise, just cover loosely and refrigerate at least 30 minutes, and up to 3 days (if it lasts that long!)