In a large bowl, whisk the egg yolks, sugar and cornstarch. In a medium saucepan, combine the milk, chocolate and salt and warm over medium heat; stir until the chocolate melts.
Gradually whisk the hot chocolate mixture into the egg mixture (1/4 cup at a time), whisking quickly, until you have warmed up the egg mixture.
Pour the egg mixture back into the saucepan and keep whisking until thickened- 12 to 15 minutes. Remove from heat; stir in the vanilla, then divide among 6 serving cups. Cover each with plastic wrap and refrigerate until well set (at least 2 hours).