Bring a large pot of salted water to a rolling boil. Add spaghetti and cook according to package instructions, approximately 8-10 minutes, until al dente. Reserve ½ cup of pasta water, then drain and set aside.

In a large skillet, melt butter over medium heat. Add minced garlic and sauté for 1 minute until fragrant. Add diced chicken and cook for 5-7 minutes until golden brown.

Lower heat and add the drained spaghetti to the skillet. Pour in the reserved pasta water and toss to create a creamy sauce that coats the spaghetti.

Plate the pasta, sprinkle with parmesan cheese and season with salt and pepper. Garnish with parsley or red pepper flakes if desired. Serve immediately.