Preheat oven to 400ºF. Lightly spray a 9x13-inch pan with cooking spray.
Spread thawed hash brown potatoes in bottom of pan.
Stir together heavy cream, salt, garlic powder (if using) and pepper. Pour over potatoes.
Bake for 30 minutes, stirring halfway through baking time.
Remove potatoes from oven, stir and top with parmesan cheese.
Bake for an additional 10 to 15 minutes, until golden and bubbly.
