Lime-Yogurt Sherbet

Directions

1. 1
   Whisk together cornstarch and 1 cup sugar in a 1 1/2- to 2-quart heavy saucepan, then slowly whisk in milk.
2. 2
   Bring to a boil over moderate heat, whisking, then simmer, whisking occasionally, 2 minutes.
3. 3
   Whisk together yolk and remaining 1/4 cup sugar in a bowl and add hot milk in a slow stream, whisking constantly.
4. 4
   Transfer to saucepan and cook over moderate heat, stirring constantly with a wooden spoon, until slightly thicker and a thermometer registers 170F.
5. 5
   (Do not let boil.)
6. 6
   Pour custard into a clean bowl.
7. 7
   Whisk in zest, juice, and yogurt.
8. 8
   Chill until cold.
9. 9
   Freeze custard in ice-cream maker.
10. 10
    Transfer to an airtight container and put in freezer to harden.
11. 11
    Serve in waffle cups if desired.