Preheat oven to 350 degrees F (175 degrees C). Butter a casserole dish.
Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes.
Drain potatoes; reserve water for mashing, if necessary. Add sour cream, cream cheese, onion salt, garlic powder, and pepper; beat until creamy. Pour into the prepared dish. Cover with Cheddar cheese, parsley, and paprika.
Bake, uncovered, until cheese is bubbly, about 35 minutes.
If refrigerating, cover with aluminum foil after finishing Step 2. Cover with Cheddar cheese, parsley, and paprika 45 to 60 minutes before serving. Bake, uncovered, for 35 minutes at 350 degrees F.