Trim the asparagus and cover in olive oil. Season well with kosher salt, garlic powder, and black pepper. Roast asparagus until slightly brown and fork tender, roughly 10-15 minutes (depending on how thick your asparagus is). Pro tip: shake the pan halfway through to evenly cook.
In a blender, add in egg yolk, lemon juice, cayenne, kosher salt, and black pepper. Pulse just until combined, roughly 3-4 times.
With the blender on, slowly add in the butter, which should be melted and piping hot. Mix until the sauce is smooth, which takes 15-30 seconds.
Serve on the side (or on top) of roasted asparagus.