Peel the sweet potatoes. Quarter the thicker ones lengthwise, then slice everything about ¼ inch thick so the pieces cook evenly over low heat.

Put the sliced sweet potatoes in a large heavy saucepan with the 6 tablespoons of butter, the heavy cream, the sugar, the salt and the halved garlic clove if using. Do not add water.

Cover and cook over low heat for 25 to 35 minutes, stirring every 10 minutes. Keep it at a bare simmer under the lid. The potatoes are ready when a slice collapses under almost no pressure.

Remove the garlic and take the pan off the heat. Mash the potatoes directly in the pan with a potato ricer, a food mill or a hand masher, working all the buttery liquid in. Never use a food processor or blender, which turns sweet potatoes gummy and cannot be undone.

Taste and add salt, then stir in the black pepper. If the mash is stiffer than you want, add warm cream one tablespoon at a time until it holds a soft peak on the spoon.

Stir in the last 2 tablespoons of butter, scatter the chives over the top and serve hot.