In separate large bowl, whisk together yolks, 1/4 cup sugar, cornstarch, and salt. Whisking the yolks constantly, slowly pour in the hot milk mixture. Return mixture to the pot and cook over medium heat, stirring constantly, until it thickens, about 3 minutes. (Do not let mixture come to a boil or it will curdle). Remove from heat immediately and strain into the bowl of chocolate. Whisk until smooth. Stir in vanilla.