makes 300ml/10fl oz
Put the egg yolks into a 900ml/ 1 1⁄2pt basin with a narrow base. Add the mustard powder. Season with the pinch of salt and pepper, and mix well using a metal spoon.
Using an electric whisk, gradually add the oil one drop at a time, whisking thoroughly between each addition. When the mixture begins to thicken, you can begin to ad slightly more oil at a time.
When about half the oil has been incorporated, add the vinegar. Pour in the remaining oil in a steady trickle, whisking all the time.
Use straight away, or store in a screwtop jar in the refrigerator for no more than a week.