In a large pot of salted boiling water, cook pasta according to package directions until al dente. Drain and return to pot.
Season chicken with salt, pepper, garlic powder, and Italian seasoning. In a large skillet over medium-high heat, heat oil, then cook chicken until browned on both sides and cooked through. Dice chicken, set aside, and cover to keep warm.
Wipe skillet clean, then melt butter over medium-high heat. Stir in garlic and cook, 1 to 2 minutes. Sprinkle flour over butter and garlic and stir until thick.
Gradually whisk in heavy cream until a smooth sauce is formed, then add salt and pepper and stir in Parmesan until melted.
Stir in chicken and penne, then top with pepper, thyme or parsley, and more Parmesan. Serve immediately.
