Cook the spaghetti in a large pot of boiling, salted water until al dente.
Meanwhile, heat a large skillet over medium heat and cook the pancetta or bacon until crispy.
Add the diced chicken to the skillet and cook until golden and cooked through, approximately 5-7 minutes.
Add minced garlic to the skillet and cook for 1-2 minutes, until fragrant.
In a bowl, whisk together the eggs, grated Parmesan cheese, and a generous amount of black pepper.
Once the spaghetti is done, reserve some pasta water before draining it.
Add the hot spaghetti to the skillet with chicken and pancetta, tossing to combine.
Remove the skillet from heat and quickly pour in the egg and cheese mixture, tossing continuously.
If the sauce is too thick, add small amounts of reserved pasta water until the desired consistency is achieved.
Season with salt and additional black pepper if desired before serving, and garnish with chopped parsley.