This recipe is used in our Rose Petal Cake, Cherry Blossom Cake, and Violet Cassis Cake.
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 the remaining sugar. Using a measuring cup or ladle, slowly pour about 1/2 cup of the hot milk mixture into the 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 saucepan, and cook, whisking constantly, until mixture returns to a simmer and thickens, about 2 minutes. Remove and discard the vanilla-bean pod.
Transfer to the bowl of an electric mixer fitted with the paddle attachment. Add butter, and beat on medium until the butter melts and mixture cools, about 5 minutes. Remove bowl from mixer, and place a piece of plastic wrap directly on the surface of the pastry cream to prevent a skin from forming. Transfer to refrigerator until cold, at least 2 hours and up to 2 days. Just before using, return pastry cream to a mixer fitted with the paddle attachment and beat until smooth, or whisk by hand until smooth.