Heat 4 cups of the milk in a large saucepan over medium heat until steaming.
Meanwhile, whisk the sugar, hot chocolate mix, cornstarch and salt in a large bowl. Whisk in the egg yolks and remaining 1/2 cup milk.
Slowly add half of the hot milk to the egg mixture, whisking constantly until smooth. Gradually whisk the egg-milk mixture into the remaining hot milk in the saucepan and bring to a boil over medium heat, whisking constantly. Continue to cook, whisking constantly, until the mixture has thickened to a pudding-like consistency, 3 to 4 minutes. Remove the saucepan from the heat and stir in the vanilla.
Let the pudding cool slightly, stirring a few times to prevent a skin from forming. Place a piece of plastic wrap directly on the surface of the pudding and refrigerate until it has completely cooled and thickened, about 4 hours.
Before serving, whisk the pudding vigorously until it's smooth and creamy. Spoon into serving bowls and top with dollops of whipped cream and a sprinkling of hot chocolate mix.
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