2 California avocados, firm but ripe (1 to 1 1/4 lb)
How to make it 1.5 hours, plus freezing
Bring 1 3/4 cups milk, 1/2 cup sugar, zest, and a pinch of salt to a simmer in a 2-quart heavy saucepan over medium heat. In a small bowl, whisk cornstarch and remaining 1/4 cup milk until smooth, then whisk into simmering milk. Bring to a boil, whisking constantly. Boil for one minute. Transfer mixture to a metal bowl, then set bowl in a larger bowl of ice and cold water and cool completely, stirring frequently. Discard zest.
Quarter, pit, and peel avocados, then purée until smooth in a food processor with remaining 1/4 cup sugar. Add milk mixture and blend well. Freeze avocado mixture in an ice cream maker, then transfer to an airtight container and freeze until hardened, about 1 hour.