Preheat oven to 425°F and line a rimmed baking sheet with foil.
Wash and scrub potatoes, then dry completely. Pierce all over with the tines of a fork, and add to prepared baking sheet.
Bake approximately 1 hour, or until fork tender.
Once baked, carefully remove sweet potatoes to a cutting board (I use tongs), and slice each potato in half lengthwise.
Reduce oven temperature to 350°F.
Scoop out the flesh of the potatoes, leaving a 1/4 inch border around the inside. Place sweet potato flesh in a mixing bowl. Be careful, as the potatoes will be very hot.
To the mixing bowl with sweet potato flesh, add brown sugar, butter, heavy cream, pumpkin pie spice and salt. Beat with a hand mixer on about MED speed until combined and fluffy.
Add the sweet potato skins back to the baking sheet, and spoon the filling from the mixing bowl into the skins.
Top with marshmallows and bake until marshmallows are as toasted as you'd like them to be. You can also broil the potatoes until the marshmallows are toasted, but you have to watch them very carefully as they can burn very quickly.
Serve sweet potatoes hot.