These are not at all something I ate growing up and I really love watching the inner kid come out in almost everyone when they see the marshmallow topping come out of the woodwork. I love the brightness the red wine vinegar brings to the party. I also think the pecans are essential for crunch.
Cook the sweet potatoes: Place the sweet potatoes in the center of the oven and bake until tender when pierced with the tip of a knife, 1 to 1 1/2 hours. Let cool slPeel the potatoes and cut them crosswise into 1-inch-thick slices. Arrange them in an 8-by-12-inch baking dish and sprinkle with salt and pepper.
Flavor the sweet potatoes: Meanwhile, in a medium pot, bring the sugar and 1/2 cup water to a simmer over medium heat. Stir in the vinegar and pumpkin pie spice and cook until syrupy, 5 to 8 minutes. Pour over the potatoes.
Bake and serve: Toss the marshmallows with the melted butter in a large bowl, use a fine-mesh strainer to sprinkle them evenly with the cayenne and arrange them on the potatoes. Bake until the marshmallows puff and brown, 10 to 15 minutes. Set the oven to broil and broil the tops for 1 to 2 minutes. Sprinkle with the pecans.