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.
Contains Raw Eggs: The Food Network Kitchen suggest caution in consuming raw and lightly cooked eggs due to the slight risk of salmonella or other food-borne illness. To reduce this risk, we recommend you use only fresh, properly refrigerated, clean grade A or AA eggs with intact shells, and avoid contact between the yolks or whites and the shell. For recipes that call for eggs that are raw or undercooked when the dish is served, use shell eggs that have been treated to destroy salmonella, by pasteurization or another approved method.
Recipe courtesy of Alton Brown