Scrape the vanilla seeds from the pod, add to the milk and bring to a gentle simmer. Whisk the egg yolks and sugar until fluffy, then whisk in the flours until smooth. Slowly pour the hot milk over the egg and sugar mix, continually whisking. Pour into a pan, and cook on a low heat, whisking until it thickens, and looks like a very thick custard. The custard needs to be thick enough to hold in the puff pastry, so cook on a very low heat for around 10 minutes, until when the whisk is removed from the custard, the ribbon it leaves stays on the surface and doesn’t disappear. Push through a sieve into a clean bowl. Cover with clingfilm, touching the surface of the custard (to prevent a skin forming). Cool completely, then chill until needed.