1. In the bowl of a food processor or blender, combine the pistachios and sugar and process in short pulses until the mixture is reduced to a fine powder. Set aside.
2. In a small bowl, combine ¼ cup of the milk with the cornstarch, and stir with a spoon to dissolve. Set aside.
3. Combine the rest of the milk (1¾ cups) with the pistachio mixture in a medium saucepan. Set over medium heat and bring just to a simmer, stirring regularly with a wooden spoon.
4. Add the cornstarch mixture and cook for 3 more minutes, stirring continuously as the mixture thickens. The custard is ready when it coats the wooden spoon, and you can trace a path on the back of the spoon with your finger.
5. Remove from the heat and let cool for a few minutes.
6. Stir in the honey and liquor and whisk to blend. Let cool to room temperature on the counter, whisking occasionally to prevent a skin from forming. Cover and refrigerate until completely chilled (overnight is best).
7. Whisk the chilled custard, and churn in your ice cream maker according to the manufacturer's directions.
*If you don't wish to use liquor, you can substitute lemon or lime juice. This will help keep the gelato soft and scoopable in the freezer.