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.