Place a fine-mesh sieve over a large heat-proof container.
In a saucepan, combine the milk and cream; heat to just under a boil.
Meanwhile, in a mixing bowl, combine the cornstarch and sugar; sift in the cocoa powder; whisk until blended.
In another bowl, whisk the eggs with the salt until blended, then add to the sugar mixture and whisk until well combined.
Slowly add half of the hot milk mixture to the egg mixture while whisking constantly.
Pour the combined egg mixture back into the pan with the rest of the milk mixture; cook over medium heat, whisking constantly, until the mixture has visibly thickened and reads 208° on a thermometer, should take about 5-7 minutes, depending on how cold your eggs are.
Immediately pour the contents of the pan through the sieve.
Add the chocolate and let the heat of the milk/egg mixture melt it.
When the chocolate has melted, blend with an immersion blender for a full 5 minutes until no lumps are visible.
Stop the blender and scrape down the sides of the container with a rubber spatula.
Immediately portion pudding into individual cups; let cool, and serve at room temperature, topped with whipped cream.
The pudding will keep, well covered, in the refrigerator for up to 4 days.