Cook the penne pasta following the package instructions, then drain and set aside.
In a large skillet, melt the butter over medium heat. Add the diced red peppers and sauté for about 1-2 minutes until slightly softened.
Stir in the minced garlic and all-purpose flour, cooking for another 1-2 minutes to form a roux.
Gradually whisk in the milk, cooking until the mixture begins to thicken. Add 2 cups of the shredded cheddar jack cheese and stir continuously until the cheese is fully melted and the sauce is creamy.
Incorporate the cooked pasta, drained chicken, and ranch seasoning powder into the skillet, mixing well. Add ¾ cup of the shredded cheese and stir until evenly distributed. Season with pepper to taste.
Sprinkle the remaining ¼ cup of cheese over the top, cover the skillet, and allow the cheese to melt. Serve warm.