Serve this classic French sauce with our Chocolate Souffle.
Whisk together yolks and sugar in a medium bowl until pale, about 4 minutes.
Pour milk and cream into a medium saucepan. Scrape in vanilla seeds; add the bean. Heat over medium heat until just about to simmer. Reduce heat to low; whisk 1/3 cup into egg-yolk mixture. Return to pan. Cook over medium heat, stirring, until thick enough to coat the back of a wooden spoon, about 8 minutes.
Pour mixture through a fine sieve into a stainless-steel bowl set in a large ice-water bath. Discard solids. Chill until cold, stirring occasionally.
You can make it two days ahead; press plastic wrap directly onto surface (to prevent a skin from forming), and refrigerate.