Combine the chopped bittersweet and milk chocolate in a medium bowl large enough to accommodate the full ice cream base. Melt the chocolate using short bursts of 20 seconds in the microwave, stirring between each interval. Once the chocolate is about three-quarters melted, stir vigorously until smooth, allowing the residual heat to finish the process. Alternatively, you can use a double boiler by placing the chocolate in a heat-safe bowl over a pan with gently simmering water. Stir frequently until smooth and silky. Add the cocoa powder and salt to the melted chocolate, mixing until fully incorporated.

In a small bowl, combine the milk powder and sugar, stirring to mix evenly. Set aside.

In a separate bowl, whisk together the cornstarch with 2 tablespoons of cold milk until completely smooth and free of lumps. Keep this mixture ready for later use.

In a medium saucepan, pour in the remaining 2¼ cups of milk, along with the heavy cream and corn syrup. Place the pan over medium-high heat and whisk occasionally to prevent the mixture from sticking to the bottom. Bring it to a rolling boil, allowing it to bubble robustly for about 1 minute.

Gradually whisk the milk powder and sugar blend into the boiling milk mixture. Reduce the heat to low, keeping it at a gentle simmer, and continue cooking for 2 minutes while whisking frequently to prevent scorching. Next, stir in the prepared cornstarch slurry and cook for an additional minute, whisking constantly until the mixture thickens slightly.

Pour one-third of the hot milk mixture into the bowl of melted chocolate, whisking thoroughly to create a smooth and glossy texture. Slowly add the rest of the milk mixture in batches, whisking continuously to ensure the chocolate emulsifies seamlessly and no streaks remain. Taking your time here is essential, as it prevents any graininess in the final ice cream.

Prepare a large bowl filled with crushed ice and a small amount of cold water to form an ice bath. Place the warm chocolate base over the ice bath and stir gently every few minutes. Allow the mixture to cool to a lukewarm temperature, which typically takes 15 to 20 minutes. Ensure the bowl remains stable to avoid spills.

Once cooled, remove the chocolate mixture from the ice bath, cover it with plastic wrap directly on the surface to prevent skin formation, and refrigerate it for at least 4 hours, or preferably overnight. A thoroughly chilled base churns into a creamier ice cream.

Pour the chilled chocolate mixture into your ice cream maker and churn according to the manufacturer’s guidelines. The ice cream is ready once it reaches a thick, soft-serve consistency and maintains its shape this usually takes 20 to 30 minutes, depending on your machine.

Transfer the freshly churned ice cream into an airtight container. Press a layer of plastic wrap directly onto the ice cream's surface to prevent ice crystal formation, then seal with a lid. Freeze for 6 to 12 hours, or until firm.

Scoop generous portions and serve immediately for a decadent, velvety chocolate ice cream experience.