Put the egg yolks into a bowl with the mustard, salt and 1 dessertspoon of wine vinegar (keep the whites to make meringues). Put the oil into a measure. Take a whisk in one hand and oil in the other and drip the oil onto the egg yolks, drop by drop, whisking at the same time. Within a minute you will notice that the mixture is beginning to thicken. When this happens you can add the oil a little faster, but don’t get too cheeky or it will suddenly curdle because the egg yolks can only absorb the oil at a certain pace. When all the oil has been added, whisk in the remaining vinegar. Taste and add a little more seasoning if necessary.
If the Mayonnaise curdles it will suddenly become quite thin, and, if left sitting, the oil will start to float to the top of the sauce. If this happens you can quite easily rectify the situation by putting another egg yolk or 1–2 tablespoons of boiling water into a clean bowl, then whisk in the curdled Mayonnaise, a half teaspoon at a time until it emulsifies again.