In a heavy saucepan stir together flour, sugar, mustard and salt.  Whisk in vinegar, egg yolks, and milk.  Cook over medium heat, stirring constantly until thickened and pudding-like in consistency (about 8-10 min.).  In bowl, beat egg whites until foamy; whisk in 1 cup of the hot dressing.  Return mixture to the saucepan and cook over low heat, whisking constantly, until mixture begins to steam again (about 1-2 min,).  Pour into clean glass jars and refrigerate up to 10 days.