1. COOK POTATOES Combine potatoes, cream, butter, sugar, salt, and pepper in Dutch oven. Cook covered, stirring occasionally, over medium heat until potatoes begin to break down, 20 to 25 minutes.
2. SIMMER POTATOES Reduce heat to medium-low and continue to cook, covered, until liquid has been absorbed and potatoes are completely tender, 15 to 20 minutes. Meanwhile, adjust oven rack to upper-middle position and heat oven to 450 degrees.
3. MASH POTATOES Add cream cheese to pot. Using potato masher, mash until cream cheese is fully incorporated and sweet potatoes are smooth or if you prefer silky-smooth potatoes, use a hand mixer to beat the potatoes. Continue to cook, stirring constantly, until potatoes are thickened, about 5 minutes.
4. TOAST MARSHMALLOWS Transfer potato mixture to 2-quart baking dish and top with single layer marshmallows. Bake until marshmallows are browned, about 5 minutes. Serve.
MAKE AHEAD: After transferring sweet potato mixture to baking dish, mixture can be refrigerated, covered, up to 2 days. Microwave until warm for 4 to 7 minutes before topping with marshmallows and baking as directed.