In a large skillet, stir-fry broccoli in butter for 3 minutes. Add squash. Cook 3 minutes longer or until the vegetables are crisp-tender. Transfer to a greased 1-qt. baking dish. In a small bowl, combine the egg, Swiss cheese, milk, mustard and cayenne. Pour over vegetables. Sprinkle with Parmesan Cheese. Bake, uncovered, at 350° for 15-20 minutes or until set.