01 - In a medium saucepan, whisk together sugar, cocoa powder, cornstarch, and salt until evenly blended.
02 - Gradually whisk in whole milk and heavy cream until the mixture is smooth and uniform.
03 - Place saucepan over medium heat and cook, whisking constantly, until the mixture thickens and begins to bubble, approximately 5 to 8 minutes.
04 - Remove from heat and immediately add chopped semi-sweet chocolate, whisking until fully melted and the mixture is smooth.
05 - Stir in pure vanilla extract thoroughly.
06 - Pour mixture into individual serving dishes, cover each surface tightly with plastic wrap to prevent skin formation, and refrigerate for at least 2 hours until set and well chilled.
07 - Serve chilled, optionally topped with whipped cream and chocolate shavings for garnish.