1 egg yolk
1/2 teaspoon fine salt
1/2 teaspoon dry mustard
2 pinches sugar
2 teaspoons fresh squeezed lemon juice
1 tablespoon white wine vinegar
1 cup oil, safflower or corn
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.