Cut chicken into evenly sized 1-inch cubes.
Season the chicken with kosher salt, black pepper, and garlic powder, then sprinkle with cornstarch and 1 tablespoon of the lemon zest and toss to coat evenly.
Heat 2 tablespoons olive oil in a large 10–12 inch skillet over medium heat.
Add the chicken in a single layer and cook until browned on one side, about 3–4 minutes, then flip and continue cooking until all sides are golden and the internal temperature reaches 165°F, about 10–12 minutes total; transfer chicken to a plate.
While the chicken cooks, 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 a drizzle of olive oil if needed and sauté the minced garlic over medium heat for about 30 seconds until fragrant.
Add the cooked pasta, 1/2 cup of the reserved pasta water, lemon juice, heavy cream, and the remaining lemon zest 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 season with additional salt and pepper if needed, then return the cooked chicken to the pan and toss to combine.
Serve immediately, topping with extra Parmesan and parsley if desired.
