For squash ribbons: Trim each end of the zucchini and summer squash; rinse and pat dry with paper towel. Using a vegetable peeler, cut wide ribbons from the whole length of the zucchini and summer squash. (Slices should be slightly less than 1/8-inch thick.) Rotate the squash, peeling all four sides to get the most color; stop peeling when you get to the seeds. Set aside ribbons; discard squash centers.
In a very large nonstick skillet, cook prosciutto over medium-high heat until crisp, about 6 minutes, turning once; drain off and discard drippings. Crumble prosciutto. Set aside.
In the same skillet, melt butter over medium heat. Add the squash ribbons and cook about 2 minutes or until just tender, turning occasionally. Sprinkle with cracked pepper; remove skillet from heat. Add the shaved Parmesan cheese; toss gently to coat. Transfer squash mixture to a serving platter and top with the crisp prosciutto.