Heat the milk in a medium saucepan over medium heat until hot but not boiling.
In a bowl, whisk together the egg, sweetener, vanilla (if using), and cornstarch until smooth.
Slowly whisk some of the hot milk into the egg mixture to temper the eggs.
Gradually whisk in the remaining hot milk.
Return everything to the saucepan.
Cook over medium-low heat, stirring constantly, until the custard thickens and begins to bubble lightly.
Remove from the heat and let the custard cool completely.
Blend the cooled custard together with the Greek yogurt until smooth using an immersion blender or food processor.
Divide the mixture evenly between two Ninja Creami pint containers.
Freeze for 12–24 hours.
Remove a pint from the freezer and run it under warm water for about one minute.
Process using the Sorbet setting on the Ninja Creami.
If adding mix-ins, make a small well in the center, add your toppings, and run the Mix-In cycle.
If the ice cream looks dry or powdery, simply run one or two Re-Spin cycles until creamy.
Serve immediately.