Chestnut Ice Cream

Directions

1. 1
   Halve vanilla bean lengthwise and scrape seeds using tip of a sharp knife into a 1 1/2- to 2-quart heavy saucepan. Add pod, cream, sugar, and chestnut puree and bring to a simmer, whisking until chestnut puree is broken up and sugar is dissolved. Remove from heat and cover pan. Let stand 15 minutes.
2. 2
   Whisk together yolks in a medium bowl, then add warm cream mixture in a slow, steady stream, whisking constantly. Pour egg mixture back into saucepan and cook over moderately low heat, stirring constantly with a wooden spoon, until custard is thick enough to coat back of spoon and registers 170°F on thermometer. Pour custard through a fine-mesh sieve into a clean metal bowl, discarding vanilla pod and forcing chestnut puree through sieve. Set bowl in a larger bowl of ice and cold water and let stand, stirring occasionally, until cold, 15 to 20 minutes.
3. 3
   Freeze custard in ice cream maker until soft-frozen, 20 to 25 minutes, then transfer to an airtight container and put in freezer to harden, about 3 hours.