In a saucepan over medium heat mix the vanilla and milk until it just starts to steam, cover and remove from the heat.

While the milk is heating fill a large mixing bowl with ice and water.

In a smaller mixing bowl combine the egg yolks, sugar, salt, cornstarch and flour. Whisk until pale and smooth. In a slow stream add the warm milk mixture whisking constantly. If your bowl is prone to sliding place a damp towel under it. Once all of the milk has been added pour the custard back into the saucepan.

Over medium-high heat whisking constantly until the pastry cream starts to thicken, about 5 minutes, keep cooking until it starts to boil. Once boiling heat for a minute before removing from the heat. It needs to cook for the full minute. Then remove from the heat.

Whisk in the butter until smooth and evenly combined. Clean out the smaller mixing bowl and place the fine mesh sieve on top. Pass the pastry cream through the fine mesh sieve to remove any lumps. Place cling film directly on top of the pastry cream to prevent a skin from forming. Place into the ice bath to cool for at least thirty minutes before refrigerating for at least 3 hours.