Heat oven to 375 degrees F. Brown and crumble meat in skillet; drain. In a medium saucepan, melt butter, add chili powder and onion and saute until onion is tender. Stir in soup, sour cream, green chilies. Mix well and remove from the heat. Reserve 3/4 cup of the sauce and set aside. To remaining sauce, add cooked meat and 1/2 of the shredded cheese; stir to combine. Along center of each tortilla, spread about 1/4 cup of the meat mixture and roll up, placing seam side down in a greased 12-by-8-inch baking dish. In a small bowl, combine reserved sauce and milk; spoon over filled tortillas. Cover dish with foil. Bake for 15 minutes, remove foil, and sprinkle with remaining cheese (and sliced black olives if desired). Bake uncovered 5 more minutes or until cheese melts.