Cook the unpeeled whole sweet potatoes in boiling water to cover for 20 to 30 minutes, until tender. Slip off the skins and mash the potatoes until smooth.
Add the nutmeg, salt, cloves, sugar, 1 tbsp of the butter, the egg, cream and grated rinds to the mashed potatoes. Beat until fluffy.
Turn the mixture into a buttered 1-quart casserole. Melt the remaining butter and brush the top of the mixture. Sprinkle with brown sugar.