In a medium saucepan, combine milk, 1/2 the sugar, vanilla and salt. Cook over medium heat until mixture comes to a simmer.
In a medium heat-proof bowl, whisk together egg yolks, cornstarch, and remaining sugar. Whisking constantly, slowly pour about 1/2 cup of the hot- mixture into the egg-yolk mixture, 1/4 cup at a time, until it has been incorporated. Pour back into saucepan, and cook over medium-high heat, whisking constantly, until it thickens and reaches 160F (71C,) about 2 minutes. Remove from heat and stir in the butter.
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 before using as cream will be quite thickened.
Notes
You can use any type of milk but whole gives the richest flavor.Do not leave mixture unintended. It can boil over quickly and will scorch or can cook the eggs within seconds.