Bring a large pot of well-salted water to a rolling boil. Cook pasta according to package directions until al dente, usually 8 to 10 minutes. Reserve 1/2 cup pasta water before draining.

While the pasta cooks, pat chicken cubes completely dry with paper towels. Season generously with salt, pepper, and half the Italian seasoning.

Melt 1 tablespoon butter in a 12-inch skillet over medium-high heat. Add chicken in a single layer and sear for 3 to 4 minutes per side until golden brown and cooked through to 165°F internally. Remove to a plate and set aside.

Reduce heat to medium. Melt remaining 2 tablespoons butter in the same skillet. Add garlic and red pepper flakes if using. Stir constantly for 30 to 60 seconds until fragrant.

Pour in chicken broth and scrape up all the browned bits from the bottom of the pan with a wooden spoon.

Stir in heavy cream and remaining Italian seasoning. Bring to a gentle simmer and cook for 2 to 3 minutes until slightly thickened.

Reduce heat to low. Gradually sprinkle in Parmesan cheese while stirring gently and constantly until the sauce is completely smooth and velvety.

Return chicken and any accumulated juices to the skillet. Add drained pasta and toss with tongs until everything is coated.

If sauce is too thick, stir in reserved pasta water one tablespoon at a time until desired consistency is reached. Taste and adjust salt and pepper. Serve immediately garnished with fresh parsley.