Preheat oven to 450°F.
Trim the woody ends from the asparagus and spread out the spears into a single layer in a shallow roasting pan or baking sheet. Drizzle with olive oil, sprinkle with salt, and roll to coat thoroughly. Roast approximately 10 minutes until lightly browned and tender, giving the pan a good shake about halfway through.
Meanwhile, put the egg yolk, lemon juice, and cayenne in a blender. Pulse to combine.
Melt the butter in a small microwave-safe bowl. 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 additional salt to taste, and serve drizzled over asparagus.