This is a technique really worth mastering. It is a great way to use leftover yolks, and prepare a mayonnaise adjusted to your flavour preferences. It keeps for quite a while in an airtight container in the fridge, and can be used as a base for various other sauces.
Mayonnaise, is an emulsion, which in very simple words means that the technique involves giving volume to a base of water soluble ingredients, using oil, which is not water soluble.
You start by preparing the base. In this case it is a solution of eggs in an acidic solvent like vinegar or citrus juice. I prefer to use only yolks, the reason being that the amount of oil required for the emulsion is less, meaning it is lighter. I also add some mustard, which apart from flavouring it, helps the emulsification. Use a standing mixer, or an electric hand-mixer, to whisk together:
Egg yolks (2 pcs is a good quantity to start)
Mild mustard, 1/2 tsp per yolk
Vinegar, 1/2 tsp per yolk
Salt, 1/4 tsp per yolk
When you have a uniform mixture, after a few seconds, start adding the oil. I like to use sunflower oil, or a mixture of a milder oil with olive oil, but not olive oil exclusively as I find the result tastes more like olive oil than mayonnaise. The oil should fall as a thin thread while whisking at a constant, medium speed. This is best done by holding your good hand at a steady angle, and it also helps to use a pouring utensil if you are not comfortable with controlling the flow from any container. The correct amount I have found is roughly 120 ml of oil for each egg yolk, but this may vary slightly depending on the size of the yolk, and the amount of the acidic solvent. The point is, if it is too loose add more oil, if it is too tight, dilute it with some more acidic solvent (lemon juice). At the end adjust seasoning (salt), and acidity, and mix in pepper if desired.
Note: If you use a whole egg, the amount of oil required, will more than double..