1. Cook pasta according to package directions in boiling salted water; drain, return to pot, and keep warm.
2. Meanwhile, cook bacon in a large skillet over medium heat until crisp. Drain on paper towels. Drain all but one tablespoon of bacon drippings from the pan.
3. Season the chicken with salt and pepper. Add the butter to the reserved bacon drippings, then add the chicken to the same skillet. Cook until tender, no longer pink, and slightly browned.
4. Sprinkle the flour and ranch dressing mix over the chicken, stirring to coat evenly. Stir in the milk, and cook, stirring occasionally, until thickened and bubbly. Stir in the cheddar cheese and half of the reserved bacon; cook and stir until the cheese is melted. Taste for seasoning and adjust as needed.
5. Add the cheese sauce to the reserved pasta