In a medium saucepan, warm 1 cup of the cream with the cocoa powder, whisking until thoroughly blended. Bring to a boil, then reduce the heat and simmer at a very low boil for 30 seconds, whisking constantly. Remove from the heat and add the chopped chocolate, stirring until smooth. Stir in the remaining cup of cream. Pour the mixture through a fine mesh sieve set over a large bowl, scraping the saucepan as thoroughly as possible.

In the same saucepan, warm the milk, sugar, and salt. In a separate medium bowl, whisk together the egg yolks. Slowly pour the warm milk into the egg yolks, whisking constantly to temper the yolks. Pour the warmed egg yolk mixture back into the saucepan. Stir the mixture over medium heat with a heatproof spatula, scraping the bottom as you stir, until the mixture thickens and coats the spatula. Pour the custard through a fine mesh sieve and stir it into the the chocolate mixture, whisking until smooth. Stir in the vanilla and liqueur. Set over an ice bath and stir until cool. Refrigerate the mixture until thoroughly chilled, several hours or overnight. Freeze it in an ice cream maker according to the manufacturer's instructions. If the chilled mixture is too thick to pour into your machine, whisk it vigorously to thin it out. Enjoy!