Whisk butter and egg yolks in large heat-resistant bowl set over medium saucepan filled with ½ inch barely simmering water (don't let bowl touch water).
Slowly add bowling water and cook, whisking constantly, until thickened and sauce registers 160-165°F, 7 to 10 minutes.
Off heat, stir in lemon juice and cayenne and season with salt to taste. Cover and set aside in warm place until serving time.
