In a medium saucepan, whisk together the sugar, cocoa powder, cornstarch and salt. Slowly drizzle in the milk and heavy cream and mix until a few dry streaks remain.
Place the saucepan on the stove and turn to medium heat. Whisk constantly until the mixture thickens and small bubbles start to form around the edges (about 10-15 minutes).
Add the chopped chocolate and stir until smooth. Turn the stove off and remove the saucepan from the heat. Stir in the vanilla extract and whisk until it is fully incorporated.
Transfer the pudding to bowls, cups or ramekins. Cool slightly, cover with plastic wrap and chill in the refrigerator for at least 1 and ? hours until set.
