01 - In a medium saucepan, whisk together the whole milk, heavy cream, and half of the granulated sugar. Heat gently over medium heat, stirring occasionally, until the mixture just begins to steam. Do not allow it to boil.
02 - In a mixing bowl, whisk the egg yolks with the remaining sugar and salt until the mixture turns pale yellow and becomes creamy in texture.
03 - Gradually pour the hot milk mixture into the yolk mixture in a slow, steady stream while whisking constantly 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 reaches 170°F on an instant-read thermometer.
05 - Remove the saucepan from heat. Add the chopped dark chocolate and unsweetened cocoa powder, stirring vigorously until the chocolate is completely melted and the mixture is smooth.
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 cooled to room temperature, whisk in the sourdough starter until it is fully incorporated and the mixture is homogeneous.
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 ice cream is thick, creamy, and has the consistency of soft serve.
10 - Transfer the churned ice cream to a lidded storage container, smoothing the top with a spatula. Freeze for at least 2 hours before serving to allow it to firm up properly.