Make this several hours before you assemble our Cherry-Blossom Princess Cake.
In a medium pan over medium heat, bring milk, vanilla seeds, salt, and 1/4 cup sugar just to boiling.
Meanwhile, whisk yolks, cornstarch, and remaining 1/4 cup sugar in a medium bowl until combined. When the milk begins to boil, remove it from the heat; slowly ladle half the milk into the egg-yolk mixture, whisking constantly. Transfer this new mixture back to the pan and return to medium heat. Continue whisking until mixture boils and thickens, about 30 seconds. Remove from heat; whisk in butter. Immediately strain into a clean bowl. Cover surface directly with plastic wrap, and let cool completely. Refrigerate at least 2 hours or overnight to set.