Cut zucchini in half lengthwise; cut a thin slice from the bottom of each with a sharp knife to allow zucchini to sit flat. Scoop out pulp, leaving 1/4-in. shells.
Place shells in an ungreased 3-qt. microwave-safe dish. Cover and microwave on high for 3 minutes or until crisp-tender; drain and set aside.
Meanwhile, in a large skillet, cook beef over medium heat until no longer pink; drain. Remove from the heat; stir in the salsa, bread crumbs, cilantro, chili powder, cumin, salt, pepper and 1/2 cup cheese. Spoon into zucchini shells.
Microwave, uncovered, on high for 4 minutes. Sprinkle with remaining cheese. Microwave 3-4 minutes longer or until cheese is melted and zucchini are tender. Serve with sour cream if desired.