1. Put the wine vinegar, water, peppercorns, bay leaf, shallot, chervil and tarragon sprigs in a small saucepan and bring to a simmer. Reduce the liquid by at least two-thirds, then strain to remove the peppercorns, bay shallot and herb sprigs.
2. To make the sauce, cut the butter into 1cm cubes. Put the egg yolks, a cube of butter and a small pinch of salt in a small bowl. Using a wooden spoon, cream the butter into the egg yolks, add ½–1 tsp of the reduction and stir to combine.
3. Half-fill a stainless steel bowl with water and set over a medium heat, to create a warm bain marie. Heat until the water is hand-hot (it should be comfortable to dip your fingers into). A bubble might occasionally rise to the surface, but if you have the water too hot, the sauce will curdle. Turn off the heat. Stand the bowl containing the egg yolk, butter and reduction mixture in the bain marie, off the heat, and stir until the egg yolks visibly start to thicken.
4. Beat the remaining butter in, a cube at a time, making sure that the sauce has re-thickened before adding the next cube of butter. As more butter is added, the heat can be turned on to low (but take care that the water does not boil) and the butter can be added more quickly. If the sauce becomes very thick and appears greasy, add a little more reduction or cool water.
5. Once all the butter has been added, remove the bowl of sauce from the roasting tin, taste it and adjust the seasoning. To achieve a balanced flavour, you may need to add more reduction, lemon juice, salt and ground white pepper. Add the chopped chervil and tarragon. The sauce can be kept for about 30 minutes before serving, by standing the bowl in a warm bain marie, off the heat.