Whisk together yolks, sugar, cornstarch, vanilla seeds, and salt in a medium bowl
. Gradually whisk in 1 cup hot half-and-half in a slow stream. Whisk egg mixture into remaining half-and-half in saucepan. Cook over low heat, stirring constantly with a wooden spoon, until thickened (do not boil) and custard registers 170 degree F on an instant-read thermometer, about 20 minutes. Immediately strain through a fine mesh sieve over a bowl. Cover surface of custard with plastic wrap and chill until cold and thickened, about 4 hours.