In a small saucepan, slowly whisk the cornstarch into 1 cup whole milk, then whisk in the sugar. Heat over medium heat for 4 to 6 minutes, whisking frequently, until it’s almost simmering and the mixture thickens.
Remove from the heat. Stir in the remaining 1/2 cup whole milk and the heavy cream, salt and vanilla extract.
Transfer the mixture to a large metal or glass bowl. Place the bowl in a smaller bowl of ice water for 1 hour, making sure not to splash any water into the custard. Alternatively, simply place the bowl in the refrigerator and wait for 3 hours or until the temperature drops to 40°F.
Churn according to your ice cream maker instructions, about 25 minutes for soft serve consistency. For hard ice cream texture, transfer to a loaf pan or sealed container and freeze in the freezer for at least 2 hours (or up to 4 hours). Store in a loaf pan with a parchment sheet over the top or a sealed container for up to 2 weeks.