Mix the flour with a little cold water, add the boiling water, boil 2 minutes and add the butter in small pieces; remove from fire and mix by degrees with the vinegar, then the mustard and the other ingredients; to this sauce the yolks of 2 eggs may be added and also the 2 whites beaten to a stiff froth; or the yolks of 2 hard boiled eggs rubbed through a sieve and the whites chopped fine and sprinkled over the salad.