Begin by melting 2 tablespoons of the butter in a large pot or Dutch oven over medium heat. Once melted and bubbly, add the cubed chicken breast. Sprinkle with salt, pepper, and garlic powder (if using). Sauté for about 5-7 minutes, or until browned and cooked through. Remove the chicken and set aside.
In the same pot, add the remaining 2 tablespoons of butter. Once melted, add the sliced mushrooms. Sauté for 4-5 minutes until tender and moisture is released.
Sprinkle the flour over the sautéed mushrooms, stirring to coat evenly. Cook for 1-2 minutes, then gradually pour in the chicken broth while stirring to prevent lumps. Add the milk, stirring until smooth and thickened.
Bring the mixture to a gentle boil. Add the uncooked pasta, stirring well. Reduce heat to low, cover, and simmer for about 10-12 minutes, stirring occasionally, until pasta is al dente and has absorbed most of the liquid.
Once the pasta is cooked, return the sautéed chicken to the pot. Stir in the Parmesan cheese until melted and creamy. Adjust seasoning with salt and pepper to taste. Serve hot, garnished with fresh chopped parsley.