In a medium saucepan, whisk together the cocoa powder, sugar, and salt. Whisk in the milk and cream. Place over medium heat and bring to a simmer, stirring frequently. Once it's hot and steaming (but not boiling), remove from heat and stir in the chopped chocolate until melted and smooth.

In a bowl, whisk the egg yolks. Slowly drizzle in about 1/2 cup of the hot chocolate mixture, whisking constantly to avoid curdling. Then, pour the tempered yolks back into the saucepan with the rest of the hot mixture.

Return the pan to medium-low heat. Stir constantly with a spatula or wooden spoon until the mixture thickens and coats the back of the spoon (170–175°F / 77–80°C). Do not boil.

Strain the custard through a fine-mesh sieve into a clean bowl. Stir in the vanilla extract. Let cool slightly, then cover and refrigerate until thoroughly chilled, preferably overnight.

Once the mixture is cold, churn it in your ice cream maker according to the manufacturer’s instructions (usually 20–25 minutes) until it reaches a soft-serve consistency.

Transfer to a lidded container and freeze for at least 4 hours or until firm.

Additions: during the final minute of churning, you can add things like fudge sauce, chopped brownies, chocolate chips, pieces of cookie dough, spoonfuls of peanut butter, etc.