Let the roast stand for about 30 minutes to come to room temperature. Pat the surface dry, then sprinkle the black pepper and garlic powder all over it and rub them in. Sprinkle about 1 cup of the Kosher salt on an aluminum foil-covered baking pan to form a thin base. Place the roast on top of this, rib side up. Preheat the oven to 325 degrees F.
Mix three cups of the salt with a little cold water, one teaspoon at a time, until it has a sandy consistency and sticks together in your hands. Start pressing it onto the surface of the Prime rib, building an even shell around 1/4th of an inch thick. If you need more salt, mix it up with a little more water until everything is covered.
Place in the oven and cook for around 20 minutes per pound. When the internal temperature reaches around 110 degrees F, turn off the oven. Leave the door closed and let the roast rest in the hot oven for 30 more minutes. Check it and remove it at 120 degrees F for rare and 125 degrees F for medium rare.
Let the roast rest for 20 to 30 minutes. Crack the salt shell and remove it in the largest pieces you can. Slice and serve immediately.