Put the egg yolks, lemon juice and a pinch of salt into your power blender.
Blend the egg yolk mixture for one minute - starting on slow and building up.
Reduce back to slow speed, slowly pour in the melted butter (through the lid plug) and blend again for a further minute. Add a tablespoon of warm water, blend again for a second or two.
Pour into a smaller jug or pan and serve.
If you don't use the sauce straight away, you can keep in warm on a very low heat and gently whisk before serving.
Serve with asparagus, blanched in hot water for three minutes.
Notes
You can add a pinch of white pepper or cayenne pepper to the sauce. You can also use the sauce to top Eggs Benedict, too.