First, make sure your ice cream maker bowl is in the freezer and chilling!
In a medium saucepan, combine the milk, heavy cream, sugar, espresso powder, vanilla extract, and salt. Cook over medium heat, stirring occasionally, until the mixture is just starting to boil.
Meanwhile, in a medium bowl, whisk together the egg yolks, additional 2 tbs. sugar, and cornstarch until the mixture is thick and pale. Once the milk mixture is boiling, slowly pour about 2 cups (no need to measure) of the hot milk into the egg yolk mixture while constantly whisking. Then, pour everything back into the pot, and cook over medium heat, whisking constantly, until the mixture is thick.
Place a fine mesh strainer over the top of a heat-safe bowl, and pour the custard through it to remove any lumps. Then cover the bowl with plastic, placing the plastic directly on top of the hot custard, and chill in the fridge for at least 4 hours, or overnight. The mixture will be thick, and should be fully chilled through.
Melt the dark chocolate in the microwave, stirring every thirty seconds until fully melted, and stir in the canola oil until combined and smooth.
Churn the ice cream according to your ice cream maker's instructions. Once it has reached about the consistency of a milkshake, start to slowly drizzle in the melted chocolate mixture with the ice cream maker still running. The chocolate will freeze and break into tiny pieces as it churns.
Pour the churned ice cream into your freezing container of choice, and freeze for at least 2 hours until hardened before serving. Enjoy!