Bring milk, 1/4 cup of sugar, vanilla seeds, and salt to a simmer in a saucepan over medium heat, whisking to disperse seeds.
Whisk the egg yolks and remaining 1/4 cup sugar in a medium bowl. Whisk in the cornstarch, 1 tablespoon at a time. Ladle a 1/2 cup hot milk mixture into the yolk mixture, whisking constantly to temper the eggs. Add the remaining milk mixture 1/2 cup at a time whisking constantly. When the milk and eggs are completely incorporated return all of the mixture to the saucepan and heat over medium-high heat, whisking constantly, until the mixture comes to a full boil and is thick enough to hold its shape when lifted with a spoon, about 2 minutes. Remove from heat and stir in the butter. *Think warm vanilla pudding consistency.
Transfer the mixture to a bowl and place plastic wrap directly onto the surface to prevent a skin from forming and refrigerate until cold. Pastry cream will keep for a few days in the refrigerator.
