1. In a medium saucepan, combine the whole milk and heavy cream over medium heat until steaming but not boiling.

2. In a separate bowl, whisk the egg yolks, sugar, cornstarch, and cocoa powder until smooth and fully combined.

3. Slowly pour a small amount of the warm milk mixture into the egg yolk mixture, whisking constantly to temper the eggs.

4. Pour the tempered egg mixture back into the saucepan with the remaining milk mixture.

5. Cook over medium-low heat, stirring constantly with a heatproof spatula or whisk, scraping the bottom and sides of the pot.

6. Continue cooking until the mixture thickens to the consistency of a loose custard that coats the back of a spoon, about 5 to 7 minutes.

7. Remove the saucepan from heat and immediately stir in the chopped bittersweet chocolate, butter, and vanilla extract until smooth.

8. Divide the hot pudding into individual serving dishes. Press plastic wrap directly on the surface to prevent skin from forming.

9. Chill in the refrigerator for at least 2 hours, preferably overnight, until fully set and chilled.

10. Serve with whipped cream and chocolate shavings for garnish, if desired.