In a medium saucepan, stir together cornstarch, cocoa powder, sugar, and salt.
Slowly stir in cold milk until combined.
Turn heat to medium and cook, stirring constantly, until mixture comes to a boil.
Boil for 1 minute, then remove from heat and stir in butter and vanilla extract.
Place a piece of plastic wrap directly over the pudding, so the pudding doesn’t form a skin on top as it cools.
Allow to cool and refrigerate for at least an hour to allow it to set up.
