Put the sour cream in a medium bowl and set aside.
In a small saucepan, combine the sugar, cornstarch, and salt. Whisk in a little of the milk until smooth, then whisk in the remaining milk. Whisk constantly over medium heat until the mixture is warm and foamy. Continue to whisk as the foam subsides and the mixture thickens and begins to boil. Whisking and scraping the bottom and sides of the pan constantly to prevent scorching, boil steadily but not furiously for 1½ to 2 minutes, or until the mixture thins slightly and becomes a bit more translucent.
It is important to cook the base adequately, or you will taste and feel the raw cornstarch on your palate and the flavor of the sour cream in the finished ice cream will not be clear and bright.
Remove the pan from the stove and whisk for a few seconds to release some heat. A little at a time, whisk the hot mixture into the sour cream. Let cool, then cover tightly and refrigerate, covered, until thoroughly chilled, at least 4 but preferably 12 hours.
For an extra-cold start, put the mixture in the freezer for 20 minutes, stirring once or twice to prevent freezing.
Freeze the ice cream according to the instructions for your ice cream machine. Serve, or pack into an airtight container and freeze. If the ice cream sits in the freezer for several hours, it will be too hard to scoop. Soften it slightly by transferring the container to the refrigerator for a few minutes before serving; or microwave on low or on the defrost setting for a few seconds at a time until scoopable.