In a small saucepan, bring 2 in. (5 cm) water to a boil. Meanwhile, in a small microwaveable bowl, heat vinegar in microwave on HIGH for 15 seconds; add cold water to vinegar. Place yolks and half of vinegar mixture in a small stainless steel bowl and whisk together in a circle or a figure eight. Don’t whisk too quickly or the yolks will get pushed up onto the sides and dry out.
Place bowl over boiling water, creating a double boiler, and turn off heat. Whisk constantly until mixture starts to thicken and becomes creamy. The mixture should run off the whisk like a strand of ribbon.
Place the bowl on the counter and continue whisking. Slowly pour in melted butter, whisk for 2 to 3 min. more. If sauce becomes too thick or shiny, add more vinegar mixture. If the yolks take a long time to incorporate with the butter, put the bowl back over the pot of water and keep whisking. Once all the butter has been incorporated, keep the sauce warm until ready to use.