Whisk together the yolks and sugar in a medium bowl until pale, about 4 minutes.
Pour the milk and cream into a medium saucepan. Scrape in the vanilla seeds; add the bean. Heat over medium heat until just about to simmer. Reduce heat to low; whisk 1/3 cup into the egg yolk mixture. Return to the pan. Cook over medium heat, stirring, until thick enough to coat the back of a wooden spoon, about 8 minutes.
Pour the mixture through a fine sieve into a stainless-steel bowl set in a large ice-water bath. Discard the solids. Chill until cold, stirring occasionally.
