Clarify butter by melting it in a very small pot or metal container over low heat and scooping out the foam that rises. Then, spoon off the clear butterfat to a clean bowl or container, being careful not to disturb the water or milk solids at the bottom. The clear portion is clarified butter. Set this aside briefly in a warm place. Combine the egg yolks, lemon juice, white pepper, and salt in a blender and process at high speed for about 60 seconds. Leaving the blender running, pour the clarified butter through the feed opening of the blender in a slow steady stream and mix until thickened. Pour into a shallow plate. Dip each asparagus spear into the sauce so that three-fourths of the stick is covered to hold prosciutto in place, then wrap a slice of prosciutto over the sauce on each asparagus spear, leaving a dry end for guests to serve themselves.