In a medium-sized saucepan, stir together 2 1/2 cups (568g) of the milk, the sugar, salt, and the vanilla bean. (If you're using vanilla extract or Vanilla Bean Crush, add it at the end.) Bring to a simmer over medium heat, stirring to dissolve the sugar., Meanwhile, whisk the cornstarch, flour, and egg yolks with the remaining 1/2 cup (113g) milk., Whisk some of the hot milk mixture with the egg yolks to temper them. This keeps the yolks from turning to scrambled eggs when you add them to the simmering milk., Pour the egg/milk mixture back into the remaining simmering milk. Doing this through a strainer will help prevent lumps later. Bring to a boil, stirring constantly with a whisk, until the mixture thickens and you see the boiling bubbles reach the center of the saucepan., Remove from the heat and strain through a fine strainer into a bowl set in an ice bath. Stir in the butter and vanilla extract. If you're going to flavor the pastry cream with chocolate or some other flavor, this is the time to do it (see variations below)., Rub a piece of butter over the surface of the cream, top with a piece of plastic wrap (make sure it touches the top of the pastry cream so it doesn't develop a skin), then refrigerate until cool., Use chilled pastry cream as is for a sliceable cream pie, or a stiff filling for éclairs. Fold in the whipped cream, just before using, for a softer filling., Pastry cream will keep, covered in the refrigerator, for up to 5 days. After that it may start to weep.