Yolk must thicken gently
Whisk yolks with acid and a little water over gentle indirect heat. The mixture should become pale and leave visible trails without forming scrambled pieces. A bowl over barely simmering water gives more control than direct high heat.
Add fat at the rate the sauce can hold
Whisk in warm melted butter very slowly at first. Once the emulsion is established, the stream can increase. Water is essential to the structure; if the sauce becomes extremely thick, a small spoonful of warm water can restore movement.
Respond early to trouble
If the bowl becomes too hot, move it off the heat and keep whisking. For a split sauce, begin in a clean bowl with a teaspoon of warm water or a fresh yolk, then slowly whisk in the broken sauce. Hold warm only briefly because both texture and safety deteriorate.