Preheat oven to 350°F (175°C). Grease a 9×13-inch baking dish.

Boil sweet potatoes in a large pot of water until fork-tender, about 15 minutes. Drain and mash until smooth.

In a large bowl, mix mashed sweet potatoes with melted butter, brown sugar, heavy cream, eggs, vanilla extract, cinnamon, nutmeg, and salt until well combined.

Spread the mixture evenly into the prepared baking dish.

In a separate bowl, mix brown sugar, flour, and softened butter until crumbly. Stir in chopped pecans.

Sprinkle the topping evenly over the sweet potato mixture.

Bake for 25 minutes, then remove from the oven and sprinkle with mini marshmallows (if using).

Return to the oven and bake for an additional 10 minutes, or until the marshmallows are golden brown.

Let cool for 5 minutes before serving.