In a medium size saucepan bring the milk to the boil

Place the egg yolks, sugar and vanilla into a a large mixing bowl and whisk

Add the flour and whisk in to incorporate fully

Gradually add the milk while whisking continuously

Pour the mixture back into the saucepan and over a medium heat, bring to the boil.

Whisk constantly for 1 minute, to achieve a smooth consistency

Immediately pour into a glass bowl and sprinkle caster sugar over the top to prevent a skin forming

Leave to cool and then cover with cling film and refrigerate until needed