Step 1: In a medium saucepan, whisk together the milk, sugar, salt, and cocoa powder. Heat over medium heat, stirring frequently, until the mixture begins to simmer.
Step 2: While the milk mixture is heating, whisk the egg yolks in a separate medium bowl until smooth.
Step 3: Slowly pour about half of the hot milk mixture into the egg yolks, whisking constantly to temper the eggs. Pour the egg mixture back into the saucepan.
Step 4: Cook the custard over low heat, stirring constantly with a heat-proof spatula, until the mixture thickens slightly and coats the back of a spoon (about 170°F). Do not let it boil.
Step 5: Remove from heat and immediately stir in the chopped bittersweet chocolate and vanilla extract. Stir until the chocolate is completely melted and the mixture is smooth.
Step 6: Strain the mixture through a fine-mesh sieve into a clean bowl. Cover with plastic wrap, pressing it directly onto the surface of the custard to prevent a skin from forming. Refrigerate for at least 6 hours or overnight.
Step 7: Once chilled, churn the mixture in your ice cream maker according to the manufacturer’s instructions. Transfer the soft ice cream to an airtight container and freeze for at least 4 hours to firm up.