200ml melted and skimmed unsalted butter (see Secrets for success, below)
squeeze lemon juice
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.