Melt the butter in a pan and remove the white foam from the top and strain the yellow liquid into a bowl.
Put the vinegar in a pan with two tablespoons of water and the pepper. Boil rapidly until the liquid has reduced to 1 teaspoon Remove from the heat and stand in a roasting tin half filled with water [ a bain marie].
Stir 1 tablespoon of water into the reduced liquid. Then add the egg yolks one at a time, whisking vigorously after each addition.
Whisk in the butter a little at a time then whisk in the remaining water. The consistency of the sauce should be like thick cream.
Pass the sauce through a sieve and then whisk in the lemon juice and salt to taste.