In a medium saucepan, whisk together the sugar, cocoa powder, cornstarch, and salt.
Gradually add the milk while whisking to combine the dry ingredients.
Place the saucepan over medium heat and cook, stirring constantly.
Once the mixture starts to thicken, reduce the heat to low.
Add the butter and vanilla extract, stirring until melted.
Continue to cook for an additional 2-3 minutes until the pudding is thick and creamy.
Remove from heat and pour the pudding into individual serving dishes.
Cover each dish with plastic wrap to prevent a skin from forming.
Chill in the refrigerator for at least 2 hours.
Serve chilled, garnished with whipped cream or fresh berries.