Prethermomix days I would make hollandaise sauce in advance. (My favourite meal in the world is salmon with hollandaise, going back to my late teens when I lived in Canada, in a University residence. My dad would meet me sometimes and take me out for dinner and I would always have smoked salmon, followed by salmon with hollandaise!) I used to put little dabs of extra butter (!) on top and then gently re-heat as Bluesed suggests then whisk the extra butter in. This would stop a skin forming on top of the sauce.
These days due to the high fat content of hollandaise and middle age spread on my part, hollandaise and salmon is a rare treat! I have only made it once since having Clancy and then I did it just before serving, as it is so easy, no stirring over simmering water etc!
