Directions

1. Put butter in a small saucepan and heat over medium-low until completely melted. Don't stir! You want butter to separate into a milky layer on the bottom and a layer of butterfat on top.
2. Put egg yolks, lemon juice, and water into blender. Pulse to blend.
3. Add lemon zest and salt to blender and process for 1 minute, pausing a couple of times to scrape down sides of blender with a rubber spatula.
4. Gently reheat melted butter until hot, about 160° (check with an instant-read thermometer). With blender running, slowly drizzle in warm butter, stopping when only milky residue is left; it's okay if some liquid pours into blender, but too much can upset the fat-to-water ratio of the emulsion and cause the sauce to separate. Blend until hollandaise is light yellow, thick, and creamy, 1 minute. Discard remaining milky residue.
5. Hollandaise is a delicate sauce, but you can keep it warm for an hour by keeping it in a thermos jar. Serve sauce on steamed asparagus or broccoli, baked halibut or sole, or Eggs Benedict.