01 - In a medium saucepan, whisk together the whole milk, heavy cream, and half of the granulated sugar. Warm gently over medium heat until the mixture just begins to steam, making sure it does not come to a boil.
02 - In a mixing bowl, vigorously whisk the egg yolks with the remaining sugar and salt until the mixture turns pale yellow and becomes thick and creamy.
03 - Slowly pour the hot milk mixture into the egg yolk bowl in a thin, steady stream while whisking constantly to prevent the yolks from scrambling. Once fully combined, return the tempered mixture to the saucepan.
04 - Cook over low heat, stirring continuously with a spatula or wooden spoon, until the custard thickens and coats the back of a spoon. The internal temperature should reach 170°F on an instant-read thermometer.
05 - Remove the saucepan from heat and immediately add the chopped dark chocolate and unsweetened cocoa powder. Stir until the chocolate is completely melted and the mixture is smooth and glossy.
06 - Pour the mixture through a fine mesh sieve into a clean bowl to remove any lumps or cooked egg bits. Allow the base to cool to room temperature, stirring occasionally to prevent a skin from forming.
07 - Once the base has reached room temperature, whisk in the sourdough starter until it is fully incorporated and no streaks remain.
08 - Cover the bowl tightly and refrigerate the ice cream base for at least 4 hours, or preferably overnight, until it is thoroughly chilled.
09 - Pour the chilled base into an ice cream maker and churn according to the manufacturer's instructions until the mixture reaches a thick, soft-serve consistency.
10 - Transfer the churned ice cream into a lidded storage container, smoothing the top with a spatula. Freeze for at least 2 hours or until firm enough to scoop before serving.