Have ready a small amount of boiling water. Yolks, vinegar or lemon juice, and oil should all be at room temperature.
Whip the yolks, vinegar or lemon juice, and salt in a bowl until light and frothy.
While whipping vigorously, begin adding room-temperature oil, 1 drop at a time, until the emulsion begins to catch (A).
Gradually slow the speed of whipping to stirring, and increase the rate at which you add the oil (B). When the mixture becomes very thick (C), stir in a few drops of lemon juice, vinegar, or cool water, and then resume adding oil and liquid until the desired texture is achieved.
Taste and correct the seasoning, adding salt, more vinegar or lemon juice, and finely ground white pepper if desired.