Preheat the oven to 375 degrees F.

Pierce the sweet potatoes several times with the tines of a fork and transfer them to a rimmed baking sheet lined with foil. Bake until tender, about 45 minutes.

Using oven mitts or pot holders, remove the sweet potatoes from the oven and set them  aside to cool for at least 1 hour before handling.

Make sure the oven rack is in the center position and preheat the oven to 350 degrees F.

Place the potatoes on a cutting board and cut in half crosswise. Gently but firmly squeeze each potato to remove the meat from the skin. Discard the skins and place the potatoes in a large mixing bowl.

Add 2 tablespoons of the butter, the heavy cream, 1/4 cup of the brown sugar, the orange juice, cinnamon, allspice, and salt to the potatoes, and mix well with a large wire whisk until smooth.

In a separate bowl, combine the remaining 2 tablespoons of butter, the remaining 1 cup of brown sugar, and the pecan pieces. Stir with a fork to blend well.

Spoon the mashed sweet-potato mixture into a 9-inch cake pan. Dot the top evenly with the pecan mixture, then arrange the marshmallows over the nuts.

Bake until the marshmallows are golden brown, about 30 minutes.

Using the mitts or pot holders, remove the pan from the oven and let it rest for 10 minutes before serving.