Preheat your oven to 375°F and line a large baking sheet with foil. Wash and prick the sweet potatoes several times with a fork to allow steam to escape during roasting.
Arrange the sweet potatoes on the prepared baking sheet and roast for 25-30 minutes until completely tender when pierced with a knife. Once cooled slightly, peel away the skin and cut into chunks for easier blending.
Add roasted sweet potato chunks to a food processor along with butter, heavy cream, brown sugar, vanilla extract, cinnamon, nutmeg, salt, and black pepper. Pulse until the mixture reaches a smooth, creamy consistency without any lumps remaining.
Transfer the blended mixture to a buttered 9x13-inch baking dish, spreading evenly across the bottom. This creamy buttery sweet potato casserole base should be about 2 inches thick and ready for the topping.
In a small bowl, combine chopped pecans with maple syrup and a pinch of salt, stirring until the nuts are evenly coated. Sprinkle this mixture over the casserole filling, then dot the top with marshmallows for a classic touch.
Bake for 10-12 minutes until the marshmallows are light golden brown and the filling is heated through. Let cool for 5 minutes before serving to allow the buttery sweet potato side to set slightly.