This section is from the book "A New Book Of Cookery", by Fannie Merritt Farmer. See also: How to Cook Everything: 2,000 Simple Recipes for Great Food.
To three finely chopped shallots add two tablespoons tarragon vinegar; bring to the boiling point and let simmer until reduced one-half. Strain, add gradually to two egg yolks, slightly beaten, and cook slowly, while stirring constantly, four minutes; then add five tablespoons melted butter, one-fourth teaspoon salt, one-eighth teaspoon cayenne and one-half teaspoon finely chopped parsley.
3 tablespoons water.
3 tablespoons tarragon vinegar.
Yolks 4 eggs 1/2 teaspoon salt 1/8 teaspoon paprika.
4 tablespoons butter.
Put water, vinegar and onion in small saucepan and heat to boiling-point. Beat yolks of eggs slightly and pour on gradually hot liquid, from which onion has been removed; then add seasonings. Place saucepan in larger saucepan, containing boiling water, and cook, stirring constantly, until mixture thickens slightly; then add butter which has been worked until creamy, one tablespoon at a time, stirring constantly during the entire cooking.