Salt
Boil the vinegar, water, and salt and butter all together in a bright saucepan. Beat the yolks a little, add some of the boiling liquid and stir them together, then mix all and let come to the boiling point. It is on the same principal as making custard. If allowed to thoroughly boil it will curdle, and have to be thrown away; if taken off too soon it will not be rich and thick enough. One minute of stirring after the yolks are added is usually enough. Strain and set on ice to be cold for use.
The preceding will be found eminently satisfactory and can be used in almost every place that mayonaise would be employed. It can be made thicker or thinner as required, but when thinned must be rapidly beaten and the liquid--water or broth and vinegar--added by degrees, otherwise the dressing may separate and the butter come to the top. This, however, seldom occurs. It is generally an object to have mayonaise creamy white, for which purpose lemon juice is added to it, but the hollandaise dressing is golden yellow, and the better for vegetable salads like the following, which have no good looks of their own.