Place the milk, half the sugar and the vanilla bean in a saucepan over medium heat.
Combine the egg yolks and the remaining sugar in a bowl and whisk until light in colour. Add in the flour and the salt, mix to combine.
When the milk just begins to boil, remove from heat and remove vanilla bean. Very slowly dribble the hot milk into the yolk mixture, stirring all the time. When about half of the milk has been added, place all of the yolk mixture back into the saucepan over medium heat. Using a spatula or a whisk, mix the pastry cream as it heats, making sure to reach all of the corners of the pan when you stir. Bring the mixture to a boil. Let boil for about 1 minute, stirring constantly. The mixture will be thick.
Remove from heat and add the butter. Strain if you wish for a smoother cream. Place into a bowl and cover directly with plastic wrap to stop a skin from forming on the cream. Use immediately or chill and use within a few days.