Peel potatoes and cut into quarters. Place potatoes in a 5- to 6-quart pan and add enough water to cover. Cover pan and bring to a boil over high heat, then simmer until potatoes are tender when pierced, 15 to 20 minutes.
Meanwhile, in a 1- to 1 1/2-quart pan over low heat, combine garlic, cream, and butter. Cover and stir occasionally until garlic is tender when pierced, about 10 minutes.
In a blender or food processor, purée garlic mixture.
Drain potatoes, return to pan, and add garlic purèe. Mash with a potato masher or mixer until smooth. Add salt and pepper to taste.