In a large cooking pot, melt 2 tablespoons of butter over medium high heat. Add the sliced mushrooms and chicken, and cook until the chicken changes color and is no longer pink (this will take about 5 minutes). Add the salt, pepper and garlic powder and mix. Take out the chicken and mushrooms and set aside.

Add the remaining 2 tbsp of butter to the pot and 1/4 cup of flour. Add the chicken stock and milk and continue whisking to ensure no lumps are formed in the sauce. Keep whisking until the sauce thickens.

Once the sauce has thickened, turn down the heat to low and add the pasta and the chicken and mushrooms back to the pot. Stir well. Cover and cook for 20 to 25 minutes until the pasta is al dente, stirring every few minutes to avoid the pasta clumping together. After the pasta is cooked, remove from heat.

Sprinkle the pasta with parmesan cheese and chopped parsley.