Preheat the oven to 350 degrees F (175 degrees C). Grease a 2-quart casserole dish.

Place potatoes in a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, 10 to 12 minutes. Drain.

Transfer potatoes to a large mixing bowl and mash until smooth. Stir in cream cheese, sour cream, onion salt, salt, and pepper. Beat with an electric mixer until light and fluffy. Transfer mixture to the prepared casserole dish and dot with butter.

Bake in the preheated oven until heated through, 30 minutes.