1/2 vanilla bean

11 oz Mexican chocolate (3 1/2 disks; preferably Ibarra), coarsely chopped

3 3/4 cups half-and-half

3 large eggs

Scant 1/4 tsp salt

Halve vanilla bean lengthwise and scrape seeds into a 3 qt heavy saucepan.  Add chocolate and half-and-half and bring to a boil over moderate heat, whisking.  Remove from heat.

Lightly beat eggs with salt in a bowl, then add hot chocolate mixture in a slow stream, whisking.  Transfer custard to cleaned saucepan and cook over moderately low heat, stirring constantly with a wooden spoon, until custard registers 175 F on thermometer, 1 to 5 minutes.  Immediately pour through a fine mesh sieve into a metal bowl.  Put bowl in a larger bowl of ice and cold water and cool, stirring occasionally.

Freeze custard in ice cream maker.  Transfer ice cream to an airtight container and freeze until hardened, about 1 hour.