In a dutch oven over medium heat, combine the sweet potatoes, heavy cream, butter, sugar, 1 1/2 teaspoons salt, and 3/4 teaspoon pepper. Bring to a simmer and cook covered until potatoes are soft and begin to breakdown, about 30 minutes.
Reduce the heat to medium-low and cook the potatoes covered until the potatoes are tender and the liquid has been absorbed, about 25 minutes. Meanwhile heat the oven to 450 degrees and place the rack in the upper middle position.
Add cream cheese to sweet potato mixture, and using a potato masher, mash until the cream cheese is thoroughly incorporated and the potatoes have become smooth. Stir constantly until potatoes are thickened, 5 minutes. Remove from heat and carefully spoon the mixture into a 3-quart baking dish.
Create a single layer of marshmallows on top of the sweet potato mixture. Bake until the marshmallows have browned, about 5 minutes. Serve.