Boil vinegar, wine, scallion, tarragon, pepper and salt over medium heat until
liquid is reduced to 2 tablespoons. Cool.
Place egg yolks and water in top of double
boiler. Have water in bottom of double boiler shallow enough so that it doesn't
touch upper saucepan. Bring water in bottom of double boiler to barely boiling.
Stir egg mixture constantly with wire whisk until it begins to thicken. Do not cook
Remove boiler from heat and add butter, one slice at a time, beating until
Remove top of double boiler from hot water.
Blend in wine and vinegar
Yields 1 1/2 cups.
Do not use an aluminum pan. Do not let sauce stand over heat. If prepared
ahead, serve at room temperature. If sauce curdles, slowly add 1 tablespoon boiling
water while beating with a mixer.