Set a large sieve over a medium mixing bowl. In a small mixing bowl, whisk the flour into the egg yolks a bit at a time until it’s smooth and there are no lumps in it. It will be a thick, pasty consistency. Set it aside.

In a medium saucepan, combine the milk, sugar, and salt. Heat it over medium low while stirring until the sugar has dissolved and the milk is steaming. Slowly drip it into the egg paste while stirring vigorously. Work about half of the milk into the eggs, then return all of it back to the saucepan.

Cook the milk mixture over medium low heat, stirring constantly, until it thickens and begins to bubble. Let it cook for another 2 minutes, still stirring, then pour it through the sieve, carefully stirring to help it pass through.

Stir in the vanilla and butter until it has melted. Place a sheet of plastic wrap directly on top of the pastry cream, then refrigerate it for at least 6 hours, or until the cream has completely cooled and thickened.

Use the pastry cream to fill tarts, eclairs or cream puffs, trifles, mille feuille, or simply serve with fresh fruit.