In a medium saucepan, combine milk, 1/4 cup sugar, vanilla bean and seeds, and salt. Cook over medium heat until mixture comes to a simmer.Advertisement
In a medium bowl, whisk together egg yolks, cornstarch, and remaining 1/4 cup sugar. Whisking constantly, slowly pour about 1/2 cup of the hot-milk mixture into the egg-yolk mixture, 1/2 cup at a time, until it has been incorporated. Pour mixture back into saucepan, and cook over medium-high heat, whisking constantly, until it thickens and registers 160 degrees on an instant-read thermometer, about 2 minutes. Remove and discard vanilla bean.
Transfer to the bowl of an electric mixer fitted with the paddle attachment. Add the butter, and beat on medium speed until the butter melts and the mixture cools, about 5 minutes.
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, beat on low speed until smooth (you can also whisk by hand).