Put the sweet potato halves in a large bowl and set aside. In a small saucepan, melt the butter over low heat. Add the sugar and allspice and stir until melted and combined. Pour the spiced butter over the potatoes. Season with salt and pepper. Toss to coat. Put the potatoes on a rimmed baking sheet.
Roast until the potatoes are tender, turning once, about 45 minutes depending on the size of the potatoes. Transfer to a warmed platter and drizzle over any accumulated pan juices. Taste and adjust for seasoning with salt and pepper. Serve immediately.