In a small heavy saucepan whisk together yolks, lemon juice, and sugar and cook over moderate heat, whisking constantly, about 15 minutes, or until a candy thermometer registers 175x F. (do not boil). Strain mixture through a sieve into a bowl and cool, surface covered with plastic wrap. Whisk buttermilk and zest into cooled lemon curd until combined well and freeze in an ice-cream maker. Makes about 4 cups.