Cookegg yolks in top of a double boiler over simmering water on medium heat, whipping constantly until tripled in volume.
Remove from heat and whip in butter, a little at a time, until all is used.
In a separate saucepan, combine vinegar, tarragon, shallots and peppercorns and cook until reduced by at least half.
Strain the reduction into the egg mixture, squeezing the tarragon to extract all the liquid. (There should only be a small amount, about 1 tablespoon). Stir until well blended. Place container in a pan of warm, not hot, water until serving time.