Preheat oven to 450 degrees F.
Trim the woody ends from the asparagus. Spread the spears in a single layer in a shallow roasting pan baking sheet, drizzle with olive oil, sprinkle with salt, and roll to coat thoroughly. Roast the asparagus until lightly browned and tender, about 10 minutes, giving the pan a good shake about halfway through.
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. 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.
Spread the roasted asparagus on a serving platter. Grind a generous amount of pepper over the top.
This recipe can be easily doubled. If you double the recipe, be sure to place the stems on the baking sheet so that the tips are pointed toward the center, and the bottoms are pointed out, toward the edges of the pan. Since the tips are thinner than the stems, they tend to cook faster. However, when roasting, items at the edges of the pan cooked more quickly than those in the center. Taking the time to lace them this way helps the asparagus cook evenly.
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