Melt the butter in a pot.
Whisk the egg yolks together with water to a creamy foam in a saucepan.
Remove the pan with the egg foam and whisk in the butter. First drop by drop and then with a sharper beam. Avoid whisking in the white sediment from the butter.
Whisk in salt and the squeezed lemon juice.
If the sauce has not thickened enough, you simply move the saucepan back onto the hob and whisk it up to the desired consistency. Here it is important to be vigilant so that it does not cut itself. Normally, you cook butter sauces in a water bath, but it works great to cook them in a pan directly on the stovetop.