Warm the bowl in hot water. Dry it. Add the egg yolks and beat for 1 to 2 minutes until they are thick and sticky.

Add the vinegar or lemon juice, salt, and mustard. Beat for 30 seconds more.

The egg yolks are now ready to receive the oil, and while it goes in, drop by drop, you must not stop beating until the sauce has thickened. A speed of 2 strokes per second is fast enough. You can switch hands or switch directions, it makes no difference as long as you beat constantly. Add the drops of oil with a teaspoon, or rest the lip of the bottle on the edge of the bowl. Keep your eye on the oil rather than on the sauce. Stop pouring and continue beating every 10 seconds or so, to be sure the egg yolks are absorbing the oil. After 1/3 to ½ cup of oil has been incorporated, the sauce will thicken into a very heavy cream and the crisis is over. The beating arm may rest a moment.

Then beat in the remaining oil by 1 to 2 tablespoon dollops, blending it thoroughly after each addition.

When the sauce becomes too thick and stiff, beat in drops of vinegar or lemon juice to thin it out. Then continue with the oil.

Beat the boiling water into the sauce. This is an anticurdling insurance. Season to taste.

If the sauce is not used immediately, scrape it into a small bowl and cover it closely so a skin will not form on its surface.

These directions are for a hand-beaten sauce. Exactly the same system is followed for an electric beater. Use the large bowl, and the moderately fast speed for whipping cream. Continually push the sauce into the beater blades with a rubber scraper.