Use this pistachio-flavored custard to make our meringue dessert, Ile Flottante.
Whisk yolks and sugar in a bowl until pale and thick, about 4 minutes. Set aside.
Put milk and cream in a medium saucepan, and heat over medium heat until just about to simmer. Reduce heat to low. Whisk 1/3 cup milk mixture into yolk mixture; pour mixture back into pan. Whisk in pistachio paste. Raise heat to medium. Cook, stirring occasionally, until thick enough to coat the back of a wooden spoon, about 8 minutes.
Pour mixture through a fine sieve into a stainless-steel bowl set in an ice-water bath. Let stand until cold, stirring occasionally. Creme can be refrigerated in an airtight container up to 3 days.