In a medium saucepan, combine milk, 1/4 cup sugar, vanilla-bean scrapings and pod, and salt. Place over medium heat, and bring to a simmer.
In a medium bowl, whisk together egg yolks, cornstarch, and remaining 1/4 cup sugar. Using a measuring cup or ladle, slowly pour about 1/2 cup of hot milk mixture into egg-yolk mixture, whisking constantly. Continue adding milk mixture, about 1/2 cup at a time, until it has all been added. Pour mixture back into the saucepan and cook, whisking constantly, until mixture returns to a simmer and thickens, about 2 minutes. Remove and discard the vanilla-bean pod.
Transfer mixture to the bowl of an electric mixer fitted with the paddle attachment. Add butter and beat on medium until butter melts and mixture cools, about 5 minutes. Remove bowl from mixer, and place plastic wrap directly on the surface of pastry cream to prevent a skin forming. Refrigerate until cold, at least 2 hours and up to 2 days. Just before using, remove from the refrigerator and return to a mixer fitted with the paddle attachment and beat until smooth, or whisk by hand until smooth.