In a medium saucepan, whisk together the granulated sugar, cocoa powder, cornstarch, and salt until well combined.

Gradually add the whole milk to the dry ingredients, whisking continuously to prevent lumps from forming.

Place the saucepan over medium heat and continue to whisk the mixture until it begins to thicken and bubble. This should take about 5-7 minutes.

Once the pudding thickens, remove the saucepan from the heat. Stir in the vanilla extract and unsalted butter until the butter is fully melted and incorporated.

Pour the pudding into individual serving pots or bowls. Allow them to cool slightly at room temperature before covering each pot with plastic wrap. Make sure the plastic wrap touches the surface of the pudding to prevent a skin from forming.

Refrigerate the pudding pots for at least 2 hours, or until fully chilled.

Serve the chocolate pudding pots chilled, topped with whipped cream and garnished with chocolate shavings or sprinkles if desired.