Prepare a large bowl filled with ice water, big enough to accommodate another medium bowl. 

Heat milk, cocoa powder, sugar, salt, and vanilla in a medium pot, whisking well until the mixture just begins to steam. Remove from heat, cover, and allow to steep for 10 minutes.

In a medium bowl, whisk together cream, egg yolks, and evoo. While whisking cream and egg mixture, slowly stream in heated milk mixture and keep whisking like a maniac. 

Return the mixture to the pot and place over medium heat. Cook, whisking constantly, until the mixture becomes thick and coats a spatula. 

Pour the mixture through a fine mesh strainer into a bowl. Whisk in melted chocolate and continue whisking vigorously over ice bath until cooled. Freeze the custard until firm before serving.