Heat the oven to 375 degrees F. Spray a 9×13 inch baking dish with cooking spray. Cook the penne pasta according to package directions for the minimum cooking time. Drain and set aside.
While the pasta is cooking, in a large microwavable bowl mix the cheese cubes, 1 cup of the shredded cheese and the milk. Microwave uncovered on high for 4 minutes, stir and then microwave in 30-second intervals until the cheese melts and the mixture can be easily stirred and is smooth.
Gently stir the cooked penne pasta into the cheese sauce. Pour the mixture into the baking dish.
In a large bowl mix the shredded chicken with enchilada sauce, green chiles, butter, and taco seasoning mix. Spoon over the pasta in the baking dish. Top with the remaining 1 cup of cheese.
Place into the oven and bake for 28 to 32 minutes or until bubbling at the edges and an internal temperature of 165 degrees F. in the center. Remove from the oven. Let stand for 5 minutes. Top with green onions.