1In a medium saucepan, combine the heavy cream, whole milk, and granulated sugar. Heat over medium heat, stirring occasionally, until the mixture is hot and the sugar is dissolved.
In a separate bowl, whisk together the cocoa powder and egg yolks until smooth.
Gradually add a small amount of the hot cream mixture to the egg yolk mixture, whisking constantly to temper the eggs.
Pour the tempered egg mixture back into the saucepan with the remaining cream mixture.
Cook over medium heat, stirring constantly, until the mixture thickens and coats the back of a spoon.
Remove from heat and strain the mixture through a fine-mesh sieve into a clean bowl.
Let the mixture cool to room temperature, then cover and refrigerate for at least 4 hours or overnight.
Once chilled, churn the mixture in an ice cream maker according to the manufacturer's instructions.
Transfer the churned ice cream to an airtight container and freeze for at least 2 hours before serving.