Place potatoes in a large saucepan and cover with water; add seasoned salt. Bring to a boil. Reduce heat; cover and cook for 15-20 minutes or until tender.
Meanwhile, in a small skillet, melt 4 tablespoons butter.
Add onion; cook until slightly softened.
Sprinkle with brown sugar; cook until brown sugar is bubbly and onion is tender.
Drain potatoes; transfer to a large bowl.
Add the sour cream, half-and-half, cream cheese, salt, pepper, cayenne and remaining butter; mash until smooth. Stir in the onion mixture.