6 large egg yolks
2/3 cup sugar
2 cups half-and-half
1 vanilla bean; split lengthwise
Whisk egg yolks and 2/3 cup sugar in medium bowl to blend. Place 2 cups half-and-half in heavy medium saucepan. Scrape in seeds from vanilla bean; add bean. Bring mixture to simmer over medium heat. Remove from heat.
Gradually whisk hot half-and-half mixture into egg yolk mixture.
Return mixture to saucepan. Stir over medium-low heat until custard thickens slightly and leaves path on back of spoon when finger is drawn across, about 12 minutes (do not boil).
Discard vanilla bean.
Cover and refrigerate until cold. (Can be prepared 1 day ahead; keep refrigerated.)