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 begins to steam, avoiding a full boil.
02 - In a mixing bowl, vigorously whisk the egg yolks with the remaining sugar and salt until the mixture turns pale yellow and reaches a creamy consistency.
03 - Slowly pour the hot dairy mixture into the yolk mixture in a thin stream, whisking continuously to prevent the eggs from scrambling. Return the combined mixture to the saucepan.
04 - Cook over low heat, stirring constantly with a spatula or wooden spoon, until the custard thickens enough to coat the back of a spoon and registers 170°F on an instant-read thermometer.
05 - Remove the saucepan from heat. Add the chopped dark chocolate and unsweetened cocoa powder, stirring 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.
07 - Once the base has cooled, whisk in the sourdough starter until it is fully incorporated and no streaks remain.
08 - Cover the bowl tightly and refrigerate for at least 4 hours, or preferably overnight, to allow the flavors to develop and the base to reach a thoroughly chilled state.
09 - Pour the chilled base into an ice cream maker and churn according to the manufacturer's instructions until the mixture achieves a thick, creamy consistency.
10 - Transfer the churned ice cream to a lidded storage container, smoothing the top with a spatula. Freeze for at least 2 hours before scooping and serving.