In a medium saucepan, heat the whole milk, cream, sugar, mint leaves, vanilla extract, and salt until it comes to a boil. Remove from the heat, and cover. Let the mixture steep for an hour, (or more if you'd like a stronger mint flavor).
Strain out the mint leaves. Reheat the milk to boiling. While heating the milk, in a small bowl, whisk together the egg yolks, cornstarch, and additional sugar. Once the milk mixture is hot again, pour about 1/2 cup into the egg yolk mixture and whisk immediately. Stir the egg yolk mixture into the remaining hot milk, and return to the heat. Whisk constantly for 4-5 minutes, or until the mixture becomes thick.
Run the mixture through a mesh siv to remove any lumps. Chill until completely cooled, about 3 hours. Once cooled, churn according to your machine's instructions.
Meanwhile, melt the dark chocolate, and stir in the oil. You want the chocolate to be easy to drizzle in a thin stream without much clumping, so add a tiny bit more oil if needed.
Once the ice cream reaches a milkshake-like consistency, start to drizzle in the chocolate slowly with the machine still running. This will allow the chocolate to break up and freeze. Once the ice cream is fully churned, place into a freezer-safe container and freeze until you are ready to serve, at least two hours. Enjoy!