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.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.
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