Bring a large pot of salted water to a boil and cook the penne pasta as per the package instructions. Reserve ½ cup of the pasta water before draining, then set the pasta aside. Pat the butterflied chicken dry with a paper towel and season thoroughly with salt and pepper on both sides. In a large skillet over medium-high heat, warm the olive oil with 1 tablespoon of butter until melted. Place the chicken in the skillet and sear for 4–5 minutes undisturbed. Flip and cook for an additional 3–4 minutes until golden and the internal temperature reaches 165°F. Transfer the chicken to a board and let it rest. Reduce the skillet heat to medium, then melt the remaining tablespoon of butter. Add garlic cloves and sauté for 2–3 minutes, stirring frequently to toast them evenly without burning. Stir in the rosemary and toast for about 30 seconds. Deglaze the pan with the chicken broth, scraping up any browned bits using a wooden spatula. Pour in the warmed cream, stirring gently to combine, and let the mixture simmer for 4 minutes. Meanwhile, slice the chicken breasts diagonally. Add the sliced chicken to the sauce, reduce the heat to low, and simmer gently for 5 minutes. Incorporate the cooked pasta, then gradually stir in ¼ cup of the reserved pasta water to loosen the sauce, adding more if needed. Toss everything until evenly coated and heated through.