Preheat the oven to 375F. Grease a 9x13-inch baking dish. Bring a large pot of water to boil.
Lower heat to med-high. Ladle the cubes of sweet potatoes carefully into the bubbling water. Cook until fork-tender, 15 to 20 min. Drain.
Add cooked potato cube to large bowl with melted butter, milk/cream, brown sugar, cinnamon, and salt. Beat with a hand mixer until smooth.
Spread mixture into baking dish. Set aside.
In a medium bowl: Combine softened butter, brown sugar, flour, and cinnamon until crumbly; stir in pecans. Sprinkle over whipped sweet potatoes.
Bake for 12 min. Remove from the oven and top with marshmallows around the edges (if using). Bake for another 5 min until golden brown.
Serve hot.
Keep covered and chilled and it should be good in the fridge for 3 days. Reheat only portions at a time; reheating repeatedly makes it watery.