Pierce sweet potatoes with knife of fork. Bake in preheated oven for 1 hour, turning once half way though cooking process.
Remove sweet potatoes from oven and allow to cool. Once cool enough to handle, carefully cut in half lengthwise. Try not to allow the skin to tear.
Carefully spoon out as much flesh as you can while leaving about a 1/4 inch thick layer against the skin.
In large bowl, mash flesh with butter, pecans, brown sugar, and pumpkin pie spice. Spoon filling back into skins. Top with equal amounts of marshmallows. Set on parchment lined baking sheet.
Set sweet potatoes back in oven and cook until heated through and marshmallows are golden brown, about 20 minutes.
I tried baking the sweet potatoes in my Instant Pot much like I make my baked potatoes, however that was a recipe fail. You see, for successful twice baked potatoes, you must have a dry skin. The Instant Pot added too much moisture and the skin just peeled away. I ended up using those for a different recipe and started all over again.
Sweet potatoes tend to be long and skinny. I highly recommend using thick, round sweet potatoes if you're wanting to twice bake them. It might take some sorting through the pile in the produce section, but they are much easier to work with for this recipe.
Feel free to experiment with the types and quantities of your mix-ins. Some other favorites you might consider mixing in could be crushed pineapple, dried cranberries, or more butter.