Put the egg yolk, lemon juice, and cayenne in a blender. Pulse a couple times to combine.
Put the butter in a small microwave-safe bowl and melt in a microwave until just melted. With the blender running, gradually add the melted butter into the egg to make a smooth frothy sauce. If the sauce is very thick, blend in a teaspoon of lukewarm water to loosen it up. Season with the salt and serve immediately or keep warm in a small heat-proof bowl set over hot (but not simmering) water until ready to serve.
Copyright 2005 Television Food Network, G.P. All rights reserved.