Submerge the pistachios in boiling water briefly, then peel off their skins. Lightly coat them with pistachio oil, ensuring even distribution. Spread them out on a parchment-lined baking sheet and place in a cold oven. Set the oven to 200°F (90°C) and allow the pistachios to dry out slowly. This process, which takes roughly 3 hours, removes excess moisture and develops a delicate toasted flavor without darkening them excessively. Since ovens and the moisture content of pistachios vary, monitor closely to prevent over-toasting. The pistachios should feel firm and slightly hardened, with a subtle golden hue in spots.

Once toasted, allow the pistachios to cool before roughly chopping them. Combine them with the milk and cream in a 3-quart saucier. Bring to a gentle simmer over medium heat, stirring occasionally. Remove from heat, cover, and let cool to room temperature. Refrigerate for at least 12 hours, preferably up to 36 hours, to extract maximum flavor.

Reheat the infused dairy mixture until it reaches a simmer. Strain it through a fine-mesh sieve into a large bowl, pressing gently on the pistachios to extract as much liquid as possible. These pistachios can be repurposed for making pistachio paste but are not needed further in this recipe.

Allow the saucier to cool slightly before whisking together the toasted sugar, egg yolks, and salt in the same pan. Once smooth, gradually incorporate the warm pistachio-infused milk and cream, whisking continuously to create a homogenous mixture.

Set the pan over medium-low heat, stirring constantly with a flexible, heat-resistant spatula. As the mixture begins to warm, increase the heat to medium, continuing to stir until it thickens slightly and reaches around 165°F (74°C). The custard should be steaming but not boiling. Strain the mixture into a non-reactive container and whisk in the chosen liqueur.

The natural color of pistachio ice cream can vary from muted green to earthy khaki, influenced by the toasting process and the richness of the egg yolks. If a more vibrant hue is desired, a tiny amount of blue gel paste can help neutralize warm tones. Using the edge of a knife, transfer a minuscule amount of gel to a whisk or spatula, then stir into the custard. Add cautiously, blending well after each addition to prevent an overly artificial appearance.

Cover the ice cream base and refrigerate until fully chilled, no warmer than 39°F (4°C). Alternatively, accelerate the cooling process by placing the container in an ice bath, stirring occasionally. Once thoroughly chilled, churn in an ice cream maker until it reaches the consistency of soft-serve.

Meanwhile, place a 1-quart storage container and a flexible spatula in the freezer to prepare for transferring the ice cream. When the ice cream is thick and airy, turn off the machine and use the chilled spatula to transfer it to the container. Enjoy immediately as soft-serve, or press plastic wrap directly onto the surface before sealing the container with a lid. Freeze until firm.

Stored properly in an airtight container with plastic wrap covering the surface, the ice cream remains at peak quality for up to one month.