Pat chicken cutlets dry. Mix salt, pepper, and garlic powder and season both sides of the chicken.
Bring a large pot of salted water to a boil. Cook penne according to package directions until al dente. Reserve 1 cup of pasta water, then drain.
Heat olive oil in a large skillet over medium-high heat. Add chicken and cook 3-4 minutes per side until golden and cooked through (internal temp 165°F). Transfer to a plate and cover loosely with foil.
In the same skillet, reduce heat to medium. Add 2 tablespoons butter and the minced garlic. Sauté 30-60 seconds until fragrant, scraping up any browned bits from the chicken.
Sprinkle flour over the garlic butter and whisk continuously for 1 minute to cook out the raw flour taste.
Slowly pour in heavy cream and milk while whisking. Add red pepper flakes if using. Bring to a gentle simmer, whisking often, until thickened, about 3-4 minutes.
Remove skillet from heat. Stir in 1¼ cups Parmesan, remaining 2 tablespoons butter, and salt and pepper to taste. If sauce is too thick, thin with reserved pasta water a splash at a time.
Add drained pasta to the sauce and toss to coat. Slice seared chicken into strips. Divide pasta into bowls, top with chicken slices, and garnish with remaining Parmesan and fresh parsley. Serve immediately.