Preheat your oven to 350 degrees F. Grease the bottom and sides of a 9x13 inch baking dish with the softened butter. Set it aside.

Spread the thawed hash browns onto a clean kitchen towel or a thick layer of paper towels. Gather the towel and squeeze firmly over the sink to remove as much moisture as possible. This step is not optional. Wet hash browns will make the casserole watery and the center will never set properly.

In a large mixing bowl, stir together the sour cream, condensed cream of onion soup, milk, diced onion, minced garlic, salt, black pepper, and garlic powder until smooth. Add the squeezed hash browns, diced ham, and 2 cups of the shredded cheddar. Stir everything together until evenly combined.

Sprinkle the remaining 1/2 cup of shredded cheddar in an even layer over the bottom of the greased baking dish. This bottom layer will crisp up during baking and act as a barrier so the casserole releases in clean squares. Pour the hash brown mixture over the cheese and spread it into an even layer with a spatula.

Cover the dish tightly with aluminum foil and bake for 40 minutes. The casserole needs this covered time to steam through and cook the potatoes all the way to the center.

Remove the foil and bake uncovered for another 15 minutes, until the top is golden brown and the edges are bubbling. Insert a knife in the center. If it meets little resistance and the center is hot, the casserole is done.

Let the casserole rest for 8 to 10 minutes before cutting. This rest time lets the filling firm up so you get clean, even squares instead of scoopable mush.