Preheat oven to 350°F (175°C). Spread 1 1/2 cups raw shelled pistachios on a baking sheet. Toast for 5-7 minutes, until fragrant but not browned. Let cool completely.

Once cooled, combine the toasted pistachios, neutral oil, powdered sugar, and almond extract (if using) in a food processor or high-speed blender. Process until a very smooth, creamy paste forms. This may take 5-10 minutes, scraping down the sides as needed. Set aside.

In a heavy-bottomed saucepan, combine the heavy cream, whole milk, granulated sugar, and salt. Heat over medium heat, stirring occasionally, until the mixture is hot and just barely simmering around the edges (do not boil). Remove from heat.

In a separate medium bowl, whisk the egg yolks until light yellow and slightly thickened.

Slowly temper the egg yolks: gradually pour about half of the hot cream mixture into the whisked egg yolks, whisking constantly to prevent scrambling.

Pour the tempered egg yolk mixture back into the saucepan with the remaining cream mixture.

Return the saucepan to medium-low heat. Cook, stirring constantly with a wooden spoon or spatula, until the custard thickens enough to coat the back of a spoon (it should reach about 175-180°F / 79-82°C). Do not boil.

Immediately remove the custard from heat. Stir in the vanilla extract and the prepared pistachio paste until fully combined.

Strain the custard through a fine-mesh sieve into a clean bowl set over an ice bath (a larger bowl filled with ice and a little water). This helps to stop the cooking process and ensures a smooth texture.

Stir the custard occasionally until it cools completely. Once cool, cover the bowl with plastic wrap, pressing it directly onto the surface of the custard to prevent a skin from forming. Refrigerate for at least 4 hours, or preferably overnight, until thoroughly chilled.

Once the base is thoroughly chilled, churn the mixture according to your ice cream maker's manufacturer instructions. This typically takes 20-30 minutes, until the ice cream is the consistency of soft-serve.

If using, gently fold in the roughly chopped pistachios during the last few minutes of churning or layer them into your freezer container.

Transfer the soft ice cream to an airtight freezer-safe container. Press plastic wrap directly onto the surface of the ice cream and then seal the container.

Freeze for at least 2-4 hours, or until firm, before serving.