In a bowl, combine the sugar, flour and cornstarch. Add the eggs, whisking until the mixture is smooth. Slowly whisk in the hot milk and vanilla bean. Return the mixture to the saucepan. Bring to a boil over medium heat, whisking constantly and scraping the bottom and corners of the saucepan to prevent sticking. Reduce the heat and simmer for about 1 minute. Remove the vanilla pod. (If using vanilla extract, add it after removing pastry cream from the heat.) To keep a skin from forming, lay a piece of plastic wrap directly on the surface of the hot cream. Let cool. Refrigerate until well chilled, about 3 hours.