In a heavy saucepan, whisk the egg yolks with the créme fraîche, butter, whisky and fish stock over a low heat until the sauce is thick enough to coat the back of a spoon - about 5 minutes. (Note: the sauce will curdle if it gets too hot.) Take it from the heat and season with salt and cayenne pepper.
The sauce can be kept in a tepid bain marie for up to 15 minutes.
