Beat together egg yolks and sugar in a medium bowl until very thick and pale, about 3 minutes.
Pour cream into a heavy, medium saucepan. Scrape seeds from vanilla bean with the point of a small knife, and add seeds and pod halves to cream. Bring cream to a simmer over medium heat; remove from heat as soon as bubbles begin to form around edge of saucepan.
Slowly whisk hot cream mixture into yolk mixture. Return mixture to saucepan, and heat over medium-low heat. Stir constantly with a wooden spoon, making sure to stir over the entire bottom of the pan and into the corners, until sauce thickens enough to coat the back of the spoon, about 3 minutes. When done, a track drawn through the custard on the back of the spoon will not close.
Stir in buttermilk.
Remove vanilla bean.
Serve warm, or place pan in a large ice-filled bowl for 25 minutes or until mixture cools to room temperature, stirring occasionally; refrigerate until cold. The sauce can be made 1 day ahead.
