Boil the pasta. Bring a large pot of salted water to a rolling boil.
Cook the pasta to al dente according to package directions. Reserve about 1/2 cup of pasta water, then drain and set aside.
Crisp the bacon. In a large skillet over medium heat, cook the chopped bacon until crispy, 6–8 minutes. Transfer bacon to a paper towel–lined plate.
Leave 1–2 tablespoons of bacon drippings in the pan.
Season the chicken. Pat the chicken dry. Toss with salt, pepper, and garlic powder. If the pan looks dry, add olive oil.
Brown the chicken. Add chicken to the skillet in an even layer.
Cook without moving for 2–3 minutes to get color, then stir and cook until just cooked through, 5–7 minutes total. Transfer to a plate.
Sauté garlic. Lower heat slightly. Add minced garlic to the same skillet and cook until fragrant, about 30 seconds.
Do not brown it.
Build the sauce. Pour in chicken broth and scrape up browned bits. Stir in heavy cream and ranch seasoning. Simmer gently for 3–4 minutes until slightly thickened.
Melt in the cheese. Whisk in Parmesan and butter until smooth.
If using mozzarella, add it now and stir until melted. Add red pepper flakes if you like a little heat.
Combine. Return chicken and half the bacon to the pan. Add cooked pasta and toss to coat.
If the sauce seems thick, stir in splashes of reserved pasta water until it’s silky and glossy.
Taste and adjust. Season with extra salt and pepper if needed. Ranch mixes vary in saltiness, so taste before adding more.
Finish and serve. Top with remaining bacon and sprinkle with parsley or chives. Serve hot, with extra Parmesan on the side.