Heat the milk, cream, and vanilla bean (with the seeds scraped into the milk) until just simmering in a heavy-bottomed medium pot. Whisk the yolks, cornstarch, sugar, and salt together and stream the hot milk mixture, while whisking continuously, into the eggs. Return to the pan and cook, while whisking over medium heat, until the mixture thickens and just begins to boil. Immediately transfer to a clean bowl. Pick out and discard vanilla bean. Stir in the vanilla extract and the butter. Press plastic wrap against the surface to cover. Chill and serve with whipped cream, if desired.