Cook the pasta: Bring a large pot of salted water to a boil and cook the pasta according to the package directions until al dente.
Reserve pasta water: Before draining, save about ½ cup of the pasta cooking water.
Season the chicken: Cut the chicken into bite-sized pieces and season with salt and black pepper.
Brown the chicken: Heat the olive oil in a large skillet over medium-high heat. Add the chicken and cook for about 5 to 7 minutes, stirring occasionally, until golden and cooked through.
Set the chicken aside: Transfer the cooked chicken to a plate and keep it nearby.
Melt the butter: Reduce the skillet heat to medium and add the butter.
Cook the garlic: Add the minced garlic and cook for about 30 seconds to 1 minute, stirring often. Do not let the garlic become dark or bitter.
Add the seasoning: Stir in the Italian seasoning and red pepper flakes if using.
Add the pasta: Add the drained pasta directly to the skillet and toss it through the garlic butter.
Add pasta water: Pour in a small splash of reserved pasta water and toss again. Add more as needed until the butter creates a smooth coating around the pasta.
Add Parmesan: Sprinkle in the Parmesan and toss until it melts into the sauce.
Return the chicken: Add the cooked chicken back to the skillet and gently toss everything together.
Taste and adjust: Add more salt, black pepper, Parmesan, or red pepper flakes if needed.
Finish the pasta: Sprinkle with fresh parsley and extra Parmesan if desired.
Serve: Serve immediately while the garlic butter sauce is warm and glossy.