01 - Combine heavy cream, whole milk, sugar, cocoa powder, and salt in a medium saucepan. Warm over medium heat, whisking until sugar and cocoa dissolve and mixture steams without boiling.
02 - In a bowl, whisk egg yolks. Gradually add about 1 cup of hot cream mixture to yolks while whisking constantly to temper.
03 - Return yolk mixture to saucepan with remaining cream mixture. Cook over low heat, stirring with wooden spoon until custard thickens and coats the back of the spoon (approximately 170–175°F).
04 - Remove from heat, stir in vanilla extract, then strain custard through fine mesh sieve into clean bowl. Cool to room temperature, cover, and refrigerate at least 4 hours or overnight.
05 - Process chilled custard in ice cream maker according to manufacturer's instructions. Transfer to container, cover, and freeze until firm, minimum 2 hours.
06 - Heat heavy cream and sugar in small saucepan over medium heat until just simmering. Remove from heat, add chopped chocolate and butter, let sit 1 minute, then stir until smooth. Stir in vanilla extract and a pinch of salt.
07 - Scoop chocolate ice cream into serving bowls and drizzle generously with warm chocolate sauce.