Pat the cubed chicken breasts dry with paper towels. Season with kosher salt, pepper, and garlic powder. Toss chicken with cornstarch until well-coated.
Heat 2 tablespoons olive oil in a large skillet over medium-high heat. Cook chicken in a single layer for 5-7 minutes, turning occasionally, until golden and cooked through. Remove and set aside on paper towels.
Bring salted water to a boil in a large pot. Cook spaghetti or bucatini until al dente according to package instructions. Reserve 1 cup pasta water, then drain pasta.
In the same skillet, add more olive oil if needed. Sauté minced garlic over medium heat until fragrant, about 30 seconds. Add fresh lemon juice, heavy cream, and half the lemon zest, stirring to combine.
Return chicken to skillet and toss in the sauce. Gradually add reserved pasta water, stirring until desired sauce consistency is reached. Add cooked pasta and toss to coat.
Remove from heat. Stir in grated parmesan cheese and chopped parsley. Adjust seasoning with salt, pepper, or lemon juice as desired. Serve hot, topped with extra parmesan and parsley.