01 - In a medium saucepan, whisk together the whole milk, heavy cream, and half of the granulated sugar. Heat gently over medium heat until the mixture begins to steam, without bringing it to a boil.
02 - In a mixing bowl, whisk the egg yolks with the remaining sugar and salt until the mixture turns pale and creamy.
03 - Gradually pour the hot milk mixture into the yolk bowl in a slow stream, whisking constantly to temper the eggs. 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 until the chocolate is fully melted and the mixture is smooth.
06 - Strain the custard through a fine mesh sieve into a clean bowl to remove any lumps. Allow it to cool to room temperature.
07 - Once cooled, whisk in the sourdough starter until it is fully incorporated and the base is homogeneous.
08 - Cover the bowl and refrigerate the ice cream base for at least 4 hours, or preferably overnight, until thoroughly chilled.
09 - Pour the chilled base into an ice cream maker and churn according to the manufacturer's instructions until thick and creamy.
10 - Transfer the churned ice cream to a lidded container and freeze for at least 2 hours before serving to allow it to firm up.