1/4 teaspoon powdered tarragon or 1 tablespoon tarragon vinegar.
Mix mustard, sugar, salt, and cayenne; add yolks of eggs, and stir until thoroughly mixed, setting bowl in pan of ice-water. Add oil, at first drop by drop, stirring with a wooden spoon or wire whisk. As mixture thickens, dilute with vinegar, when oil may be added more rapidly. Keep in cool place until ready to serve, then add remaining ingredients.