Melt the butter in a cup by setting it into hot water for a few minutes and then set it aside to cool; stir the yolks in a small vessel with salt and sugar to a cream and add the melted butter, a little at a time, stirring constantly; next slowly add the vinegar, set the vessel in a saucepan of hot water and stir until the contents begin to thicken; then remove, stir until cold and slowly add the other ingredients; beat the whites to a stiff froth and mix them with the sauce; if handy add a few tablespoonfuls cream; if English mustard is used take 1 teaspoonful mixed with cold water.