Begin by boiling the shelled unsalted pistachios in a pot of water for 2 minutes. Drain and rinse them under cold water to cool down, then transfer to a clean kitchen towel. Rub the pistachios vigorously to remove their skins.

In a high-powered blender, combine the blanched pistachios and whole milk. Blend on high speed until the mixture becomes smooth and creamy, about 2-3 minutes. Transfer to a container and refrigerate overnight.

After the mixture has chilled overnight, strain through a fine mesh strainer if needed to ensure smoothness.

In a medium saucepan, gently heat the pistachio-milk mixture over low-medium heat. If too thin, mix a tablespoon of corn starch with a small amount of cold milk, then whisk into the warm mixture until thickened slightly.

Remove from heat, whisk in sugar, salt, remaining milk, and optional pistachio extract. Stir in heavy cream until fully combined.

Pour the combined mixture into a bowl or container and refrigerate for several hours or overnight.

Transfer the chilled mixture into an ice cream maker and churn according to manufacturer's instructions, about 20-30 minutes. Add coarsely ground pistachios a few minutes before churning ends.

Serve immediately for a soft-serve texture or freeze in an airtight container for at least 4 hours for a firmer gelato.