In a medium saucepan, combine sugar and cornstarch, whisking to thoroughly incorporate. Add milk and whisk until dry ingredients are dissolved. Next, whisk in the egg yolk and cream.
1/4 cup granulated sugar, 1/4 cup cornstarch, 1 1/3 cups cold milk, 1 egg yolk, 2/3 cup heavy cream
Place the pan over medium heat, whisking constantly until the mixture comes to a simmer and thickens. While it takes a while for the mixture to heat, it will go from thin to thickened in just a few minutes once it is simmering.
As soon as the mixture thickens, remove the pan from the heat and whisk in the vanilla.
1/2 teaspoon pure vanilla extract
Add chocolate and whisk until it is completely melted and smooth.
3 ounces semisweet chocolate
Pour pudding into individual serving dishes. Let the pudding cool slightly then cover the tops with plastic wrap to prevent a skin from forming. Refrigerate for at least 2 hours before serving.
If desired, garnish with fresh whipped cream and chocolate curls.
Notes
Store covered in the refrigerator for up to 3 days.