Clarify the butter by placing it in a double saucepan over simmering water until the butter has melted and cleared. Remove the scum from the top and carefully pour off the pure fat; discard the thin milky liquid left behind. Keep the fat warm.
Combine the vinegar, peppercorns and shallots in a saucepan and bring to the boil. Reduce the liquid to a quarter of its original volume and then allow to cool. Take a round-bottomed bowl and set it over a saucepan of very hot but not quite boiling water. In this, whisk together the vinegar mixture and the egg yolks with the
Once made the sauce cannot be kept warm and must be used straightaway.