1. Preheat oven to 425°F.
2. In a 9×13-inch casserole dish, add 4 cups of the chicken stock, elbow macaroni, and salt. Stir to combine; submerge pasta in liquid. Cover tightly with aluminum foil.
3. Place casserole dish in oven. Bake 40 minutes.
4. In a medium bowl toss together Cheddar, Parmesan, and 1 tablespoon of the cornstarch. Set aside. 
5. Remove baking dish from oven, remove foil. Stir pasta to separate and redistribute.
6. In a 4-cup glass measuring cup or medium microwave-safe bowl whisk together remaining 1 cup of chicken stock, remaining 1 tablespoon of cornstarch, the cream, butter, and garlic powder until smooth. Microwave on high 2 to 3 minutes, or until boiling. Stir until smooth.
7. Pour cream mixture over pasta. Add cheese, stir until melted and smooth.
8. Let stand 5 minutes before serving, sauce will thicken.

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