In a medium saucepan, combine the heavy cream, whole milk, granulated sugar, Dutch-process cocoa powder, and salt. Whisk continuously over medium heat until the sugar dissolves and the liquid is hot but not boiling, about 5-7 minutes.

In a separate bowl, whisk together the egg yolks. Gradually add about a cup of the hot cream mixture while whisking continuously to temper the yolks.

Return the tempered mixture to the saucepan and cook the custard over low heat, stirring constantly until thick enough to coat the back of a spoon, about 5-8 minutes.

Strain the custard through a fine-mesh sieve into a clean bowl to remove any curdled bits. Stir in the remaining heavy cream and vanilla extract. Cover and refrigerate until fully chilled, at least 4 hours or overnight.

Churn the chilled custard in an ice cream maker according to the manufacturer's instructions, or whip the additional cream and fold it into the custard if opting for the no-churn method.

Transfer the churned ice cream to an airtight container, smooth the surface, cover with plastic wrap, and freeze for at least 4 hours until firm.