Boil chicken and cut into bite-sized pieces. Melt butter or margarine in a 9 X 13-inch baking dish in preheated oven. Sprinkle half of muffin mix over melted butter. Cook carrots and onion in a large pot of boiling, salted water until carrots are tender. Add zucchini and cook until zucchini is hot. Drain off liquid. Add vegetables, garlic powder, soup, sour cream, chicken, salt and pepper to pot and mix to coat. Pour mixture into baking dish on top of butter and muffin mix. Sprinkle the remaining muffin mix on top of chicken mixture and bake at 350 degrees for 1 hour, uncovered.