Bring large pot of salted water to rolling boil. Cook fusilli until al dente, 10 to 12 minutes. Before draining, reserve ½ cup pasta water. Drain pasta and set aside.

Pat chicken cubes completely dry with paper towels. Season with salt, pepper, and ½ teaspoon Italian seasoning. Heat 1 tablespoon butter in large 12-inch skillet over medium-high heat until shimmering. Add chicken in single layer and sear 2 to 3 minutes per side until golden and cooked to 165°F. Transfer to plate, leaving pan juices in skillet.

Reduce heat to medium. Add remaining 2 tablespoons butter and melt. Add minced garlic and red pepper flakes if using. Stir constantly for 30 to 60 seconds until fragrant and just golden. Watch closely to prevent burning.

Pour in chicken broth and scrape up all browned bits from bottom of pan. Stir in heavy cream and remaining ½ teaspoon Italian seasoning. Simmer gently for 2 to 3 minutes until slightly thickened and reduced by about one-third.

Reduce heat to low. Gradually add Parmesan cheese ¼ cup at a time, stirring continuously until each addition melts completely. Sauce should become smooth and coat the back of a spoon.

Return chicken and any accumulated juices to skillet. Add drained pasta and toss for 1 to 2 minutes until well coated. If sauce is too thick, add reserved pasta water 2 tablespoons at a time until glossy and clingy.

Taste and adjust seasoning with salt and pepper. Remove from heat, garnish with fresh parsley, and serve immediately.