Combine mustard with water and let stand for ten minutes. Divide the butter into three parts.
Place one-third of the butter in the top part of a small double boiler. Add vinegar and egg yolks. Place over hot, not boiling, water and cook slowly, beating vigorously.
As mixture begins to thicken, add the second piece of butter, then the third, beating constantly. Stir and cook until thickened. Do not overcook. Add the mustard mixture and mix well. Add salt to taste. If the sauce tends to curdle, place pan over ice and stir vigorously.