In a large pot, cover the potatoes and garlic with water by 1 inch. Bring to a boil, add salt and simmer until tender, about 15 minutes. Drain well.
Put the potatoes and garlic back in the pot and cook over low heat to evaporate the rest of the water. Mash in the butter with a potato masher. Add the heavy cream in stages until the desired consistency is reached. Season with salt and pepper.