Hold both ends of a stalk of asparagus and bend it until it snaps. Then use a knife to trim the remainder of the bunch to match. Discard the trimmed stems.
Bring a pot of generously salted water to a boil. Add the asparagus and cook, uncovered, until tender, 4 to 6 minutes depending on the thickness. Drain in a colander.
Meanwhile, put the egg yolk, lemon juice and cayenne in a blender. Pulse a couple times to combine.
Put the butter in a small microwave proof bowl and heat 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 really thick blend in a teaspoon of lukewarm water to loosen it up.
Transfer the asparagus to a plate. Pour the sauce over the top of the asparagus, sprinkle with more cayenne if desired and serve right away.
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