In large bowl, whisk 2/3 cup of the milk, egg yolks and cornstarch until smooth.
In a saucepan, stir the remaining milk, sugar and salt until well combined.
Cook over medium heat until steaming, but do not boil.
While whisking, slowly pour about half of hot milk into egg yolk mixture to warm.
Transfer back into saucepan.
Continue stirring constantly, until the mixture thickens and boils.
Remove from heat,stir in the butter, chocolate and vanilla extract.
Cover top directly with plastic wrap and chill in refrigerator until completely cool.