The most reliable method for hollandaise is to make a mousse by whisking egg yolks with water (a tablespoon per yolk), over heat, directly or in a double boiler. The process should take at least three minutes, by which time the mousse should be light but close-textured. The mousse acts as a stabilizer so that melted butter can be added without the sauce separating. Other methods of making hollandaise, for example by heating egg yolks and cold butter in a water bath, are slower and more difficult to master.
Formerly hollandaise was seasoned with a reduction of vinegar, but nowadays lemon juice is more common. Serve hollandaise plain or with flavorings, to accompany poached fish, eggs and vegetables. Leftover hollandaise or béarnaise can be kept, covered and refrigerated, for up to two days.
Mock hollandaise (sauce bâtarde), which is included as a hollandaise variation, is not really a hollandaise at all, but is based on a roux. It can be kept warm for longer than basic hollandaise and is more economical.