Bring a large pot of salted water to a boil. Cook the penne until al dente according to the package directions. Reserve 1/2 cup of the pasta cooking water before draining, then drain and set the pasta aside.

While the water heats, slice the chicken thin, about 1/4 inch. Thinner is better here because it cooks through fast without getting dry. Season the chicken on both sides with garlic powder, 1/2 teaspoon of the salt, and the black pepper.

Heat 2 tablespoons of the olive oil in a large skillet over medium-high heat. When the oil shimmers, add the chicken in a single layer. Do not overcrowd the pan. Cook 3 to 4 minutes per side until golden and cooked through to 165 degrees F in the thickest piece. Transfer the chicken to a plate and cover loosely with foil to rest.

Add the remaining 1 tablespoon of olive oil to the same skillet. Add the zucchini and the remaining 1/4 teaspoon salt. Cook over medium-high heat for 4 to 5 minutes, stirring occasionally, until the zucchini is tender and has some color on the cut sides. Add the minced garlic and red pepper flakes and stir for 60 seconds until fragrant.

Pour the chicken broth into the skillet. Scrape up all the browned bits from the bottom of the pan with a wooden spoon. Those bits are where most of the flavor lives. Let the broth simmer for 2 to 3 minutes until it reduces by about a third.

Reduce the heat to medium-low. Add the butter and stir until melted. Sprinkle in the Parmesan gradually, stirring as you go so it melts into the sauce smoothly rather than clumping. If the sauce looks too thick, stir in a splash of the reserved pasta water.

Add the drained penne to the skillet and toss with tongs until the pasta is coated. Slice the rested chicken into pieces and return it to the pan. Toss gently to combine everything. Taste for salt and adjust. Serve in bowls with parsley and extra Parmesan on top.