Heat up the milk with the scraped vanilla bean and half of the sugar. (If using vanilla bean paste or vanilla extract, add it just when the milk comes to a boil.)
While the milk is heating up, whisk together the rest of the sugar, flour, corn starch egg, and egg yolks.
Once the milk comes to a boil, turn the heat off. Hold the pot of hot milk over the bowl of the egg mixture and start whisking the egg mixture. As you whisk, pour in approximately one-third of the milk into the egg mixture, whisking constantly.
Pour the tempered egg mixture into the pot with the rest of the heated milk. Whisk to combine.
Turn the heat to medium high and whisk the mixture constantly. Once it comes to a boil, set a timer for 2 minutes and continue whisking and cooking for exactly 2 minutes.
Immediately pour the pastry cream onto plastic wrap lined baking sheet (it's a good idea to pour it through a strainer) and enclose it tightly with the plastic wrap touching the entire surface to avoid a skin forming on the pastry cream. Allow it to cool to room temperature.
Notes
Pastry cream can be made up to 48 hours in advance and stored wrapped in plastic wrap in the refrigerator.