Whisk together the egg yolks, vanilla extract and caster sugar using an electric whisk until the mixture is pale and thick. Whisk in the plain flour.
Boil the milk, then gradually whisk into the egg mix. Return to the pan and cook over a gentle heat, whisking until the custard is thick and glossy. Cook gently, stirring for 2 mins more
to cook out the raw flour taste.
Remove from the heat and beat in the butter. Spoon into a clean bowl, then cover closely with cling film (so it's in contact with the custard) and leave to cool.
Whip the double cream, then fold into the cool
custard. Can be made up to 2 days ahead and stored in the fridge.
Just before using, stir in the creme fraiche.
Note: This custard is part of an Orange berry wedding cake recipe