Whisk the cocoa powder and half the sugar with the milk in a medium saucepan until smooth.

Add the cream and a pinch of salt; warm gently over medium heat until hot but not boiling, stirring occasionally.

Remove from heat, add the chopped dark chocolate, and whisk until fully melted and glossy. Let the mixture cool slightly for about 5 minutes.

In a separate bowl, whisk the egg yolks with the remaining sugar until the mixture lightens in color and is slightly thickened.

Slowly stream about 1/3 of the warm chocolate-milk mixture into the yolks while whisking constantly to temper the yolks.

Return the tempered yolk mixture to the saucepan and cook over low heat, stirring constantly until the custard coats the back of the spoon (170–175°F / 77–80°C).

Pour the custard through a fine-mesh sieve into a clean bowl to remove any cooked bits. Stir in the vanilla.

Cool the custard to room temperature, then cover and refrigerate until thoroughly chilled—ideally 4 hours or overnight.

Churn in an ice cream maker according to the manufacturer’s instructions until soft-serve consistency.

If adding mix-ins, fold them in during the last minute of churning.

Transfer the churned ice cream to a freezer-safe container, press a piece of parchment directly on the surface, seal, and freeze at least 4 hours or overnight to firm up.