Melt the butter in a saucepan over low heat. Heat butter to bubbling but do not let it brown.
Meanwhile, into the container of an electric blender add the remaining ingredients. Cover the container and blend on low speed. The yolks must not be over-blended or the sauce will not make. Immediately uncover and pour in the hot butter in a steady stream. When all butter is incorporated, turn off motor. Keep the sauce warm by placing the container in a saucepan with two inches of hot water. If the sauce becomes too thick, thin it with a little hot water briefly blended.