Some extra notes:
Hollandaise sauce is one of the classic French mother sauces. It is essentially an egg-emulsified butter sauce and as such a good hollandaise should have the butter as the dominant flavour and have a nuance of vinegar and lemon – not the other way round.
The sauce is very similar to mayonnaise but needs to be kept warm so the butter remains fluid. Also, it is necessary to temper the egg for a hollandaise sauce. Tempering refers to heating the egg either in a hot liquid or bringing the egg and liquid up to temperature such as to cause the egg to thicken. A temperature of under 65 deg C (150 deg F) should be maintained to prevent scrambling (or coagulation in geeky terms). The addition of acid, in the recipe here it’s the vinegar, reduces the chance of the hollandaise splitting (or curdling).
Clarified butter can be used which will result in a thicker sauce – this is because whole butter on average contains 15% water which thins the sauce, whereas clarified butter is pure butterfat.
The sauce should be served immediately and not allowed to sit for any length of time – it can develop nasty bugs if left to stand at room temperature. In a professional kitchen the sauce can be maintained at 60-65 deg C. (140-150 deg F.) for a recommended time no longer than 2 hours.
