Hollandaise Sauce is an egg-based sauce flavoured with an acid such as lemon or vinegar.
There are many different recipes and just as many opinions. The basic sauce starts as an emulsion of egg and water, to which butter is added, then something acidic to add flavour. Emulsion has to happen: if you just took melted butter and whacked in egg yolks, you wouldn’t achieve the right smooth texture. Food scientists, though, such as Harold McGee say that eggs aren’t actually needed for the emulsification to happen.
The sauce needs to be cooked over a double-boiler; but don’t let the water below actually boil, just keep it at a rolling simmer. If it goes beyond that and starts to boil, add a few tablespoons of cold water to settle it down. If the sauce separates a bit, add a tablespoon or two of cream and whisk it smooth again. If the sauce just plain curdles, haul out the blender and whiz it in there, though the texture will be affected a bit. Another way of attempting to rescue a curdled Hollandaise is to beat one egg yolk with a teaspoon of water and a quarter-teaspoon of butter in a bowl until blended, then to whisk this mixture a bit at a time into your curdled Hollandaise until it smooths out again.
If you don’t have the time to make it over a double-boiler, but don’t want to stoop as low as using a canned version or a packet mix, there are recipes for Hollandaise that can be made in a microwave or a blender. People who know how to make Hollandaise over a double-boiler say these middle-road methods are ideal if you are having the kind of day that involves picking up kids from hockey practices, etc.
Some experts advise not using a stainless steel pan. You can keep Hollandaise Sauce warm in a double-boiler for up to 1 1/2 hours, or in a pre-heated thermos for several hours. Fans of the thermos technique say that this is the best way to kept it warm safely and without risk of curdling. Most food safety people advise to toss Hollandaise after a few hours, as the risk of it turning unsafe to eat will start to increase.
Watercress, briefly blanched and then puréed into lightly-warmed sour cream or a thick, higher-fat Greek yoghurt (typical 2% fat yoghurts will be too watery.)
For safety’s sake, plan to serve the sauce as soon as it is ready.
Recorded as early as 1758 as “sauce à la hollandoise”, according to Alan Davidson in The Penguin Companion to Food, but back then it didn’t include eggs yet.