In a small bowl, mix together ¼ cup of the milk with the cornstarch.
In a medium saucepan, whisk together the remaining milk, salt, and sugar. Allow the mixture to heat over medium heat until it is steaming; do not let it boil.
While the milk is heating, whisk in the cocoa powder.Whisk the egg yolks together in a separate small bowl.
Once the milk is steaming, slowly pour the egg yolks and the cornstarch mixture into the warm milk mixture, whisking constantly.
Remove from the heat and stir in the butter and vanilla extract.
Pour into individual serving dishes. Top each with a layer of plastic wrap touching the top of the pudding to prevent a skin from forming. Refrigerate until chilled.