Have ready a small amount of boiling water. Yolks, vinegar or lemon juice, and oil should all be room temperature.
In the blender jar or processor bowl, blend or process yolks, vinegar or lemon juice, and salt until light and frothy.
With the blade running, pour in oil through the top opening or feed tube in a very thin stream. As the emulsion catches and the mixture becomes thick, begin adding oil faster and faster.
When the mixture becomes very thick, add 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.