Put 6 tablespoons water in a small saucepan over a medium heat. Add the vinegar, season with salt and pepper and simmer for 2 minutes until reduced by half and the liquid becomes syrupy. Transfer the vinegar reduction to a heatproof bowl and rest it over a saucepan of gently simmering water, making sure the bottom of the bowl does not touch the water (this is called a bainmarie). Add the egg yolks and beat the mixture continuously over a low heat until it turns white, thickens and the liquid coats the back of a spoon. Don’t let the water boil or your sauce will turn into scrambled eggs!
Now add the melted butter to the vinegar reduction a little at a time, omitting any ‘milk solids’ that form at the bottom of the pan, whisking continuously. When it starts to thicken, add 1 tablespoon water, then continue adding the butter until it is all incorporated. The mixture should be smooth and light – you may need to add a little more water to achieve this consistency. Season again with salt and pepper and keep warm in the bainmarie until ready to serve. Just before serving, squeeze in a few drops of lemon juice. Taste and add more juice if you like a stronger lemony flavour.
