In a medium saucepan, bring the milk, heavy cream and sugar to a boil, whisking constantly until the sugar is dissolved. In a medium heatproof bowl, whisk the egg yolks. Gradually whisk in half of the hot cream. Whisk the egg-and-cream mixture into the saucepan and cook over moderate heat, stirring with a wooden spoon, until the custard coats the back of the spoon, about 4 minutes. Pour the custard over the chocolate and let stand for 2 minutes, then stir until smooth.