For the best mashed sweet potatoes, we favor roasting over simmering or steaming. During roasting, moisture in the potatoes evaporates, concentrating the natural sugars for heightened sweetness. And the caramelization that occurs on the exterior contributes complex flavor even though the potatoes are peeled before mashing. Select potatoes weighing nine to 12 ounces each so they cook at the same rate. (For this recipe, pass on white, purple and Japanese sweet potatoes, as these varieties are drier and denser.) The roasted potatoes are easily mashed, but if you prefer a smoother texture, press the flesh through a ricer before stirring in the remaining ingredients. To make ahead, after incorporating the butter mixture and crème fraîche, allow the potatoes to cool, then cover and refrigerate up to three days; also refrigerate the butter reserved for garnish. When ready to serve, microwave on high, covered, for 4 to 5 minutes, stirring once. If needed, stir in a tablespoon or two of water to thin the consistency, then re-melt the reserved butter and drizzle it onto the potatoes.