Place potatoes and onion into a large pot and cover with water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain.
Transfer cooked potatoes and onion to a large bowl. Add butter, buttermilk, garlic powder, salt, and black pepper; blend together until creamy, 1 to 2 minutes.
Substitute margarine for the butter if preferred.
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