Put 6tablespoons 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 1tablespoon 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.