Whisk butter and egg yolks together in large heat-resistant bowl set over medium saucepan with 1/2 inch of barely simmering water (don’t let bowl touch water) until mixture is smooth and homogeneous.
Slowly add 1/2 cup boiling water and cook, whisking constantly, until thickened and sauce registers 160 to 165 degrees on instant-read thermometer, 7 to 10 minutes. Off heat, stir in lemon juice and cayenne. Season with salt to taste. Remove saucepan from heat (keep bowl over water bath) and season with salt to taste. Cover to keep warm.