In the bowl of a stand mixer with the whisk attachment, mix the egg yolk, mustard, vinegar, and Worcestershire sauce on medium-high speed for 2 minutes until light.
Very slowly stream in the oil to make the emulsion. Continue pouring the oil in slowly until it is thick like mayonnaise in a jar, about 4 minutes.
Turn the mixer off and add the two salts and garlic powder. Mix on medium-high speed for 1 minute more.
Refrigerate until needed (best if refrigerates at least over night) and up to 1 week. Makes 1 3/4 cup.