Place ingredients in the top of a double broiler. Place about one inch of water in the bottom part of the double boiler, and bring water to a boil. Turn the fire down to very low. Place the top part of the double boiler into the bottom part. Add 1 stick (1/2 c) of VERY COLD butter to the egg mixture. Whisk until the butter melts and the sauce thickens. Keep the sides and bottom scraped. If the water seems to be boiling too much simply lift the top part of the double boiler out of the bottom, and let it calm down.
When the sauce thickens, put a top on, and serve as soon as possible. This never curdles on me.