Preheat the oven to 425 degrees F. Roast the potatoes on a rimmed baking sheet until soft and tender, about 1 hour. Let cool for a few minutes, then use paper towels or a kitchen towel to help remove the skins.
Process the sweet potato meat, butter, 1 1/2 teaspoons salt, cinnamon, allspice and cayenne in a food processor until smooth. Let cool to room temperature. Transfer to an airtight, microwave-safe container, label and date, and freeze for up to 2 weeks.
To serve, reheat in the microwave, stirring often. Transfer to a serving dish, and top with the pecans and scallions.
To serve the potatoes right away instead of freezing them, after processing the potato mixture, transfer it to a serving dish and sprinkle with the pecans and scallions.
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