Whisk together sugar, cornstarch, and salt in a medium saucepan.
Whisk together egg yolks and vanilla seeds (if using) in a glass measuring cup. Whisk in milk
Add to saucepan along with butter and bring to a boil over medium heat. Let boil 1 minute, still whisking; then remove from heat and stir in vanilla extract (if using).
Strain pastry cream through a fine-mesh sieve into a bowl.
Cover with plastic wrap, pressing it directly onto surface of cream to prevent skin from forming. Refrigerate until chilled, at least 2 hours and up to 2 days. Just before using, whisk until smooth. Storing No-Fuss Pastry Cream needs to chill for a minimum of two hours before it can be used. It can also be made up to two days in advance.
