In a glass bowl, whisk together egg yolk and dry ingredients.
Combine lemon juice and vinegar in a separate bowl then thoroughly whisk half
into the yolk mixture. Start whisking briskly, then start adding the oil a few
drops at a time until the liquid seems to thicken and lighten a bit, (which
means you've got an emulsion on your hands).
Once you reach that point you can relax your arm a little (but just a
little) and increase the oil flow to a constant (albeit thin) stream. Once half
of the oil is in add the rest of the lemon juice mixture.
Continue whisking until all of the oil is incorporated.
Leave at room temperature for 1 to 2 hours then refrigerate for up to 1 week.