Preparation Beat together yolks, sugar, and water in a metal bowl set over a saucepan of simmering water with a hand-held electric mixer at high speed until thick, pale, and registers 140°F on thermometer. Continue beating over simmering water, maintaining 140°F, 3 minutes more. Remove bowl from heat and gradually add pistachio paste, beating until incorporated. Chill until cold, about 15 minutes. Beat cream with cleaned beaters until it just holds stiff peaks. Stir one third of cream into pistachio base to lighten, then fold in remaining cream gently but thoroughly. Scrape into an airtight container and freeze until firm, about 4 hours.
Preparation Beat together yolks, sugar, and water in a metal bowl set over a saucepan of simmering water with a hand-held electric mixer at high speed until thick, pale, and registers 140°F on thermometer. Continue beating over simmering water, maintaining 140°F, 3 minutes more. Remove bowl from heat and gradually add pistachio paste, beating until incorporated. Chill until cold, about 15 minutes. Beat cream with cleaned beaters until it just holds stiff peaks. Stir one third of cream into pistachio base to lighten, then fold in remaining cream gently but thoroughly. Scrape into an airtight container and freeze until firm, about 4 hours.