In a medium saucepan, whisk together the cream, milk, sugar and salt.
Heat over medium heat until the mixture is hot and steaming but not boiling.
In a separate bowl, whisk together the egg yolks.
Slowly add 1/2 cup of the hot cream mixture to the egg yolks, whisking constantly.
Add the egg yolk mixture back into the saucepan with the remaining cream mixture and cook over medium-low heat, stirring constantly with a wooden spoon or heatproof spatula until the custard thickens and coats the back of a spoon.
Strain through a fine mesh sieve into a bowl and stir in cocoa powder until fully incorporated.
Cover with plastic wrap directly on top of custard to prevent a skin from forming. Chill in refrigerator for at least 4 hours or overnight.
Once chilled, churn in an ice cream maker according to manufacturer's instructions. Transfer to an airtight container and freeze for at least 4 hours before serving.