Preheat your oven to 350°F (175°C).
Boil the cubed sweet potatoes in salted water until tender, about 15-20 minutes.
Drain the sweet potatoes and place them in a large mixing bowl.
Mash the sweet potatoes until smooth.
Add melted butter, brown sugar, milk, eggs, vanilla extract, cinnamon, and nutmeg to the bowl.
Mix until well combined and creamy.
Transfer the sweet potato mixture into a greased baking dish.
If desired, sprinkle mini marshmallows on top.
In a separate bowl, combine chopped pecans with a bit of melted butter and brown sugar.
Spread the pecan mixture evenly over the casserole.
Bake in the preheated oven for 30-35 minutes, or until golden brown on top.
Let cool for a few minutes before serving.