In a small saucepan, slowly whisk the cornstarch and into 1 cup of the whole milk, then whisk in the cocoa powder and 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 milk, 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 overnight, 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. Store in a loaf pan with a parchment sheet over the top or a sealed container for up to 2 weeks.