1. Whisk butter and egg yolks together in large heat-resistant bowl set over medium saucepan with 1/2 inch of barely simmering water (dont let bowl touch water) until mixture is smooth and homogeneous.

2. Slowly add 1/2 cup boiling water and cook, whisking constantly, until thickened and sauce registers 160°F on instant-read thermometer, 7 to 10 minutes. Off heat, stir in lemon juice and cayenne. Season with salt to taste.

Yield: 2 cups