Step 1

In a medium bowl, combine the sour cream, yogurt, lemon juice and zest.

Step 2

In a small saucepan, combine the sugar, cornstarch, salt and 2 tablespoons of the milk. Whisk until a uniform paste forms, then whisk in the remaining milk. Place the pan over medium heat and cook, whisking constantly until the mixture foams and boils. Take care not to burn the mixture. When the mixture thickens slightly, remove the pan from the heat.

Step 3

Gradually stir the cornstarch mixture into the sour cream mixture. Let cool, then cover and refrigerate until thoroughly chilled, preferably overnight.

Step 4

Freeze the in an ice cream machine according to the manufacturer's instructions.