Peel and roughly chop the shallot into small pieces.
Strip the mint leaves from the stalk and chop .
Crush the peppercorns with the back of a knife, so that they release their flavour.
Put the shallot, bay leaf, peppercorns, mint and vinegar into a small saucepan.
Place the saucepan over a medium heat and allow the liquid to evaporate until about half of the liquid remains.
Scrape the mixture into a sieve with a heatproof bowl underneath that you will make the sauce in.
Clarify the butter – see notes.
Set up a bain marie with the vinegar reduction – see notes.
Add the egg yolks to the strained liquid and whisk for 2 minutes over a gentle heat in the bain marie..
Continue whisking for a further 2 minutes until the mixture is pale and thick.
Whisk in a small amount of butter at first.
Then add the remaining butter in a steady stream, whisking continuously.
Turn off the heat and season to taste.
Serve warm or at room temperature.