Cut the chicken into 1-inch pieces. In a large skillet, melt butter over medium heat and cook the chicken until slightly browned.
Add minced garlic and sauté for another minute.
Pour in uncooked penne and chicken broth. Stir to loosen any brown bits from the skillet and bring to a boil. Cover, reduce to medium-low heat, and simmer for about 8 minutes, until pasta is tender.
Stir in milk, cream cheese, and pesto, cooking until the cream cheese is melted.
Mix in Parmesan cheese until fully integrated.
If desired, add spinach and sun-dried tomatoes, cooking until the spinach wilts.
Season with cracked black pepper and crushed red pepper. Serve hot.