http://www.marthastewart.com/1084932/no-fuss-pastry-cream
Whisk together sugar, cornstarch, and salt in a medium saucepan. Whisk together milk and egg yolks in a glass measuring cup. Add milk mixture to the saucepan, along with butter. Cook over medium heat until mixture comes to a boil. Let boil 1 minute. Remove from heat and add vanilla.
Strain pastry cream through a fine-mesh sieve into a bowl. Cover with plastic wrap, pressing it directly onto the surface of the pastry cream to prevent a skin from forming. Refrigerate until chilled, at least 2 hours or up to 2 days. Just before using, whisk until smooth.