Cut the chicken into evenly sized cubes and place in a bowl; season with kosher salt, black pepper, and garlic powder.
Add the cornstarch and 1 tablespoon of the lemon zest to the chicken and toss until evenly coated.
Heat 2 to 3 tablespoons olive oil in a large (10–12 inch) skillet over medium heat.
Arrange the chicken in a single layer in the skillet and cook without moving until browned on one side, about 3–4 minutes.
Turn the pieces and continue to cook, browning all sides, about 10–12 minutes total, until the chicken reaches 165°F; transfer to a plate.
Meanwhile, bring a large pot of salted water to a boil and cook the spaghetti or bucatini according to package directions; reserve 1 cup of pasta water before draining.
To the same skillet, add the minced garlic and cook 30 seconds, adding a small drizzle of oil if the pan is dry.
Add the cooked pasta, 1/2 cup of the reserved pasta water, lemon juice, remaining lemon zest, and the heavy cream to the skillet; toss and stir for 1–2 minutes until the pasta is evenly coated, adding more pasta water as needed for a light, slightly creamy sauce.
Stir in the grated Parmesan and chopped parsley, taste and adjust seasoning if needed, then return the cooked chicken to the pan and toss to combine.
Serve immediately, topping with additional Parmesan and parsley if desired.