Put the chocolate in a heatproof bowl and set it over a saucepan of simmering water. Let the chocolate melt, stirring until smooth. Set aside to cool.
Beat the egg yolks and sugar in an electric mixer until thickened and pale, about 5 minutes on medium-high speed.
Meanwhile, combine the cream, milk, cocoa and salt in a saucepan. Bring the mixture just to a boil - when little bubbles form around the edges of the pan, you're done. Transfer to a container with a pouring spout (like a Pyrex liquid measure) and slowly drizzle into the egg mixture on low speed.
Return the mixture to the saucepan and cook for 5 - 7 minutes over medium-low heat, whisking constantly, until thickened enough that the whisk leaves a trail in the pan. If you want to be sure, the temperature should read 175 degrees on an instant thermometer.
Whisk the melted chocolate into the custard until completely smooth. Strain through a fine mesh strainer into a bowl. Cover and refrigerate until very cold - at least 6 hours or overnight.
Scrape the mixture into an ice cream maker (it will be thick) and churn according to directions. You can serve right away, or transfer to a container to freeze for a few more hours. Scoop truffle-sized servings and dust with cocoa powder.