Set a large pot of cold water to boil. Once it reaches a rolling boil, add a pinch of salt and let return to a full boil.
After washing the asparagus and cutting away any woody ends, boil them in the water just until you can penetrate the thick end of 1 of the stalks by stabbing it with a fork. Lift out the asparagus but continue to let the water boil. Cut off the tips of the asparagus and reserve. Chop the asparagus stalks into 1-inch pieces and then puree either in a manual food mill or in a blender or food processor. The result should be a thick cream.
Melt the butter in a saucepan and then add the prosciutto bits, sauteing just until they give off a little fragrance. Then stir in the asparagus puree and, moments later, the Parmigiano-Reggiano. Cook gently, taking care that the sauce does not boil.
At this point, toss the penne into the pot of boiling water. Cook for 8 minutes or according to package instructions. Once penne is cooked, drain water and place penne in a warm bowl.
While the penne is cooking, add the asparagus tips and some of the cream to the sauce. You do not want to make the sauce too liquid, so only add as much cream as will give the sauce a velvety texture.
Once the penne is cooked, spoon the sauce on and toss. Serve on individual plates with a pinch more Parmigiano-Reggiano on top an
Recipe courtesy of Fred Plotkin, author Italy for the Gourmet Traveler: Recipes from Paradise