Like any custard, you do need to be sure to follow the instructions to make sure that it doesn't curdle. But it is pretty stable and we have never had a problem. The key is to keep stirring and not leave it on its own. Also, to remove it from the heat as soon as it is done.
It is possible to make it with a hand whisk, but it is much easier if you have an electric whisk.
This quantity is quite small - enough to make six apple tarts. It is admittedly harder to cook in a small quantity - but if you're only making six tarts its less wasteful.
You could always make a double quantity and use half to make vanilla slices for afternoon tea and the rest to make apple tarts for dessert one night.
Using an electric mixer on medium speed, whisk the egg yolks and 1/3 of the sugar together until the mixture forms ribbons when the beaters are lifted. This should take 3 or 4 minutes. Beat in the flour. You will now have a thick pale yellow mixture, almost a paste.
Heat the milk gently along with the rest of the sugar and 1/2 tsp of vanilla extract. Once the sugar is completely dissolved, you can bring it to the boil.
As soon as the milk comes to the boil, pour it onto the egg yolk mixture, stirring all the time to avoid curdling. It is really important to keep stirring the whole time.
Pour the mixture back into the pan and heat on low to medium. Keep stirring with a whisk as you heat it. It will quite suddenly become thick and creamy. Reduce the heat immediately and cook out the flour for a minute or two, but keep it moving all the time with the whisk.
Remove from the heat and pour straight into a bowl. Once it has cooled slightly, the pastry cream is ready to use. If you are not using it immediately, dust with icing sugar, or dot with unsalted butter, or cover with baking parchment (which needs to actually touch the pastry cream) to prevent a skin forming and refrigerate until needed.
It will keep for three or four days in the fridge.