This is one of my favorite sauces.
Although many claim that it is hard to do because it can easily "go crazy", this never happened to me, since the first time I have cooked it.
It is only a question of patience proceeding carefully and without haste.
The best method of cooking in a bain-marie, but you can also cook directly over a vry low flame using an heat spreader.
Ingredients for 6: 1cl of Water, 125 gr of Butter, the juice of 1 Lemon, 2 Egg yolks, Salt and White pepper.