Mix the sugar, vanilla sugar and egg yolks in a bowl. Add the flour and mix well.
Heat the milk in a saucepan. As soon as it starts to boil, remove from the heat. You can tell it is starting to boil because bubbles will form at the edge of the pan.
Pour some of the milk into the egg mixture and stir until the milk is completely absorbed. Then add the rest of the milk gradually, stirring until you have a smooth mixture.
Pour the mixture back into the pan and turn up the heat. Bring to the boil, continuing to stir, making sure that the bottom of the pan is covered. The cream is ready when it has the consistency of a thick custard.
Pour the cream into a clean bowl and cover with plastic wrap. Press the foil on the cream to prevent a skin forming on the cream.
Leave to cool for a while, then place in the fridge to cool completely. The cream will continue to thicken.
How to store
You can store the pastry cream in an airtight container for several days.