In a medium saucepan, whisk together the sugar, cornstarch, cocoa powder and salt.
In a medium sized bowl, whisk together the egg yolks, milk and heavy cream. Scrape the vanilla seeds into the bowl and add the vanilla bean pod.
Add the egg yolk mixture into the saucepan (with the cocoa powder mixture) and keep whisking until the mixture is smooth.
Turn the stove on to medium high and constantly whisk the mixture until it starts to boil. Reduce the heat to medium and keep whisking for another ~4 minutes or until the mixture becomes very thick (enough to coat the back of a spatula). It should resemble a slightly thinner version of regular pudding.
Take the saucepan off the heat and turn off the stove. Whisk in the melted chocolate, butter and vanilla extract. Keep whisking until the mixture thickens to the consistency of pudding.
Strain the pudding through a fine mesh sieve into a large bowl. Then transfer the pudding into individual jars, bowls or containers. Cover the pudding with plastic wrap and make sure to put the plastic wrap directly on top of the pudding so a skin does not form during the cooling process. Place the pudding into the refrigerator and allow to cool for at least 4 hours.
Serve with fresh whipped cream or crushed pretzels if desired.
Keep the pudding covered and chilled in the refrigerator. It will keep in the refrigerator for several days.Source: Bon Appetit magazine, January 2014 issue, page 12