Put the milk to boil with the sliced vanilla pod and its seeds.
Meanwhile, put the yolks in a bowl with the sugar. Beat it energetically, until the mix gets creamy.
Add the flour and mix well.
Once the milk has boiled, add it gradually to the previous mix. Then put everything back in the pot, and put it back on a medium heat.
Stir constantly with a wooden spoon, until the cream thickens, as it is wanted. This lasts around 10 minutes. There you are, your pastry cream is ready !