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Boil the vinegar together with peppercorns and tarragon, reduce by half. Strain and reserve (see Secrets for success on storing, below).
Boil a large pan of water, then reduce to a simmer. Using a large balloon whisk, beat together the yolks and 2 tsp of the reduced wine vinegar in a heatproof bowl that fits snugly over the pan.
Beat vigorously until the mixture forms a foam, but make sure that it doesn’t get too hot. To prevent the sauce from overheating, take it on and off the heat while you whisk, scraping around the sides with a plastic spatula. The aim is to achieve a golden, airy foam (called a sabayon), which forms ribbons when the whisk is lifted.
Whisk in a small ladle of the warmed butter, a little at a time, then return the bowl over a gentle heat to cook a little more. Remove from the heat again and whisk in another ladle of butter. Repeat until all the butter is incorporated and you have a texture as thick as mayonnaise. Finally, whisk in lemon juice, salt and pepper to taste plus a little warm water from the pan if the mixture is too thick.
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