Put the vinegar, shallots, peppercorns if using and half the tarragon into a heavy-bottomed stainless-steel or enamel saucepan and bring to the boil. Reduce rapidly for 4 minutes. Pass through a sieve into a bowl and return to the saucepan. Allow to cool.
Melt the butter in a small saucepan. Add the egg yolks to the vinegar reduction and beat with a spiral whisk. (a balloon whisk is lik