Whisk together the egg yolks and half of the sugar.
In a saucepan, combine the milk, cream, remaining sugar, cocoa powder and salt.
Heat over medium heat until steaming, whisking until the cocoa powder has fully dissolved.
Slowly whisk the hot cream mixture into the egg yolks.
Return the mixture to the saucepan.
Cook over medium-low heat, stirring constantly, until the custard reaches 170–175°F (77–79°C).
Remove from the heat and stir in the chopped dark chocolate and vanilla until completely smooth.
Strain through a fine-mesh sieve.
Cover and chill for at least 4 hours, preferably overnight.