01 - In a medium saucepan, thoroughly whisk together the granulated sugar, unsweetened cocoa powder, cornstarch, and salt.
02 - Gradually whisk in the whole milk and heavy cream until the mixture is smooth and completely combined, free of lumps.
03 - Place the saucepan over medium heat and cook, continuously whisking, until the mixture begins to noticeably thicken and gently bubble. This process typically takes about 5 to 7 minutes.
04 - Reduce the heat to low. Add the finely chopped chocolate and butter, whisking until both are fully melted and the pudding achieves a smooth, glossy consistency.
05 - Remove the saucepan from the heat and stir in the vanilla extract.
06 - Pour the warm pudding into individual serving dishes. Cover each dish with plastic wrap, pressing it directly onto the surface of the pudding to prevent a skin from forming as it cools.
07 - Refrigerate for at least 45 minutes to chill thoroughly, or serve immediately while warm if preferred.