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.
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. Here is where you'll whisk in the optional liqueur flavoring.
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. [*Note: This is an optional step; it makes the cream a bit more stable at room temp and counters the corn-starchy/gelatin mouth feel, but can be omitted if you're looking to cut some fat].
Cover with plastic wrap, pressing it directly onto the surface of the pastry cream to prevent a skin from forming. Allow to come to room temperature, then 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).