1. In a large pot of boiling salted water, cook macaroni until tender but still firm, about 8 minutes. Drain in a colander.
2. Meanwhile, sprinkle chicken with chili powder. In a large frying pan, melt 2 tablespoons of butter over medium heat. Add chicken and onion and cook, stirring occasionally, until chicken is lightly browned and onion is golden, about 7 minutes. With tongs, remove chicken from pan.
3. Melt remaining butter in pan. Stir in flour to make a paste. Cook, stirring, 1 to 2 minutes. Slowly whisk in milk. Bring to a boil, stirring until sauce thickens, 3 to 4 minutes. Remove from heat and stir in cheese until melted. Stir in chiles and chicken. Add sauce to macaroni, stir to blend, and serve