Place sugar in a large bowl. Sift over it the cornstarch, cocoa and salt. Add 1/2 cup of the milk and stir the mixture to make a thick paste. Lightly beat the egg yolks, then add these to the cornstarch mixture, whisking to blend well. Wrap a damp towel around the base of the bowl to prevent it from sliding around when the scalded milk is added.
Meanwhile, in a heavy-bottomed 2-quart saucepan, combine the remaining 2 cups milk and the cream. Bring these just to the boil, then remove the pot from the heat. Pour a small amount of the hot liquid into the cornstarch-cocoa mixture, whisking constantly. Continue to whisk the milk-cream mixture into the bowl gradually, until all of the liquid has been incorporated, and the mixture is smooth. Rinse out the pot used to scald the milk and the cream, but don't dry it: this will help prevent the pudding from scorching on the bottom. Pour the custard into the clean pot and add the vanilla.
Have ready a clean mesh sieve over a medium sized bowl. Return the pot to the stove and stir with a wooden spoon over low to medium-low heat until the custard thickens, about 5 to 7 minutes. It should approach but never quite reach, a boil and be about the consistency of mayonnaise when it's done. This custard behaves strangely, you may fear something has gone terribly wrong, but press on! It will get increasingly lumpy to the point where, just as it reaches the right thickness, it will seem downright chunky: never mind! Quickly remove the pot from the element and pour through the sieve into the clean bowl, pressing the custard through with a rubber spatula.
Add the finely chopped or grated chocolate in 2 additions, stirring gently with a clean wooden spoon or rubber spatula until the chocolate is melted smoothly into the pudding. Spoon the pudding into 6 to 8 serving dishes or goblets and chill at least 6 hours or overnight. If you like the pudding with no skin on the top, press plastic wrap onto the surface of the warm pudding in the serving dishes. If a skin on your chocolate pudding makes you happily nostalgic, wait until the puddings are cold before covering. The pudding can be made up to 2 days ahead of time.
Serve just as it is, or with a dollop of lightly sweetened whipped cream for a truly decadent dessert.