Place the cubed sweet potatoes in a large pot and cover with water. Boil until fork-tender.

While the potatoes boil, preheat your oven to 350°F (175°C).

Drain the sweet potatoes and return them to the pot. Mash until smooth.

Add the brown sugar, milk, melted butter, vanilla extract, cinnamon, and nutmeg to the mashed sweet potatoes and mix well.

Spread the sweet potato mixture evenly in a greased baking dish.

In a separate bowl, combine chopped pecans and marshmallows (if using) for the topping.

Sprinkle the pecan mixture evenly over the sweet potato layer.

Place the dish in the preheated oven and bake for 30 minutes until golden and bubbly.