Preheat the oven to 400 degrees F. Pierce the sweet potatoes 2 or 3 times with a fork. Bake in a roasting pan for 45 to 60 minutes, until tender. Cut the potatoes in half, lengthwise. Hold the sweet potatoes with a potholder or towel and scoop out the potato flesh into a food mill or ricer.
In a medium saucepan, heat the butter, syrup, zest, and vinegar over medium heat. Remove from the heat and mash the sweet potatoes in the pan. Stir to combine evenly. Season to taste with salt and pepper. Serve immediately.
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