1. In a medium saucepan, combine milk, 1/2 cup sugar, vanilla bean and seeds, and salt. Cook over medium heat until mixture comes to a simmer.

2. In a medium bowl, whisk together egg yolks, cornstarch, and remaining 1/2 cup sugar. Whisking constantly, slowly pour about 1 cup of the hot-milk mixture into the egg-yolk mixture. Continue adding milk mixture, 1 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 begins to boil, about 2 minutes. Remove and discard vanilla bean.

3. Transfer to the bowl of an electric mixer fitted with the paddle attachment. Add butter; beat on medium speed until butter melts and mixture cools, about 5 minutes.

4. Cover with plastic wrap, pressing it directly onto 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).