Bring a pot of salted water to a boil. Trim the asparagus. Place the asparagus in the water and blanch for about 4 to 6 minutes. Remove the asparagus from the water and season with salt and pepper. While the asparagus are blanching, prepare the sauce. In a stainless steel bowl set over a pot of simmering water over medium heat, whisk the egg yolks with the lemon juice, water, mustard, and parsley, together. Season with salt and cayenne. Whisk the mixture until pale yellow and slightly thick. Be careful not to let the bowl touch the water. Remove the bowl from the pot and whisking vigorously, add the butter, 1 teaspoon at a time, until all is incorporated. Place the asparagus on a platter and spoon the Hollandaise sauce over the top.