In a bowl, mix together the sugar, cornstarch, and cocoa powder.

Pour the milk into a saucepan and add the dry mixture; stir well so that no lumps form.

Place the pan over low to medium heat and warm, stirring, until the mixture begins to thicken gently.

Finely chop the chocolate.

As soon as the mixture thickens, remove it from the heat and add the chopped chocolate, vanilla extract, and salt; keep stirring until the chocolate is completely melted and the pudding is smooth.

Spoon the pudding into a serving bowl. Let it cool slightly, then refrigerate for at least 4 hours (preferably longer) to set completely.

Serve cold, optionally topped with a little whipped cream.