Chop the chicken breasts into equal sized cubes. Season all over with the kosher salt and pepper.
Season all over with the garlic powder. Sprinkle on the cornstarch and one tablespoon of the lemon zest and toss well.
Heat the olive oil in a large (10 or 12 inch) skillet over medium heat. Add the chicken in a single layer.
Meanwhile, bring a pot of salted water to a boil and cook the pasta according to the directions. Make sure to reserve 1 cup of pasta water once the pasta has cooked.
Turn the chicken after it’s browned on one side, about 3 to 4 minutes. Continue to brown all over, for a total of about 10 to 12 minutes, until the internal temperature is 165 degrees F. Transfer the chicken to a plate.
To the same skillet, add the garlic and cook for 30 seconds (if needed, add another drizzle of olive oil here).
Add in the cooked pasta, ½ cup pasta water, lemon juice, the heavy cream, and remaining lemon zest. Toss and stir for a minute or two so the pasta gets completely incorporated.
Add more pasta water if needed. We’re not making a thick sauce, but just more of a slightly creamy lemon coating for the spaghetti. It will still be very light.
Add the parmesan cheese and the parsley. Toss well again. Taste and season more if needed. Add the chicken back in the pan.
Serve immediately. Top with extra parmesan and parsley.
