1. Beat together egg yolks and sugar in a medium bowl until very thick and pale, about 3 minutes.
2. 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.
3. 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.
4. 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.